Relative Error

It isn't at all unusual for siblings to be close to each other. Lots of siblings will hang out with each other and show some sibling affection towards each other. If someone else sees them and knows that they're related, they won't raise any eyebrows.

If they do not know that they're related, they might think the two are dating.

Oftentimes, the one who catches the two together is a lover or love interest of one of the siblings, and thus thinks that he/she is cheating or doesn't like her/him. Drama and/or hilarity ensues.

This trope is often used as a type of The Reveal where the audience does not know that the two siblings are related beforehand, and are led to believe that the person is cheating or likes someone else.

And, of course, this is most definitely Truth in Television.

This is somewhat the inverse of, but definitely comparable to, Like Brother and Sister. If the characters themselves don't know, it's Surprise Incest. If their resemblance to a romantic couple is particularly convincing, there may be Incest Subtext at work. Sometimes though, it happens to be Exactly What It Looks Like.

Warning: Spoilers Ahead.

Anime and Manga
"Shinichiro: (to his guy friend) That's him! That number four. He's Noe's... Noe: Brother! Shinichiro: ...brother. (pause) Brother?!"
 * True Tears, episode 3: Shinichiro sees Noe ride off on a motorcycle with another guy, who he assumes is her boyfriend. He finds out otherwise when he takes her to a basketball game and sees the same guy...


 * Used as the final reveal in the Di Gi Charat Winter Garden spinoff.
 * School Rumble plays with this one: After Harima sees Tenma and Karasuma together, he sulks in his room and ends up watching "Comet Wars", which mirrors the Star Wars use of this trope. Harima suddenly realizes he may have made this mistake and that Karasuma is Tenma's brother... but later realizes he was just fooling himself.
 * Not quite, but also in it Harima has a cousin named Itoko. Because the Japanese word for cousin is itoko, every time he says "My cousin" it sounds like he's saying "My Itoko", as if the two are dating. This causes mass confusion.
 * In Love Monster, Hiyo assumes that Kurou is flirting with Chiya, but in reality he just found out Chiya is his sister. Later on in the story, a squicky situation occurs where Chiya does actually try to pursue a relationship with Kurou, which is perfectly acceptable in monster society. Kurou is not interested however.
 * Used in Dragonball Z. Krillin mistakenly believes that Android 17 and 18 are together.
 * In Baka and Test Akihisa attempts to keep his sister who has just recently moved home hidden from his friends for fear of her embarrassing him. When they start getting suspicious and go to his house they believe that he has a secret live in girlfriend, much to Himeji's denial:
 * "Akihisa-kun, this bra is way too big for you. You need a smaller cup size."
 * In Weiss Kreuz, Omi and Sakura both assume that Aya-chan is their Aya's lover when they find out he's been taking care of a girl in a coma and using her name. The Assassin and White Shaman manga has a scene where Aya-chan forces her brother to leave his job early and buy her something to eat, and his coworker, implying that this has happened before, assumes she's his girlfriend.
 * Uragiri wa Boku no Namae wo Shitteiru: Toko and Tsukumo Murasame seem to fit this, even going so far as to sleep in the same bed.
 * Uragiri wa Boku no Namae wo Shitteiru: Toko and Tsukumo Murasame seem to fit this, even going so far as to sleep in the same bed.

Comic Books

 * This, along with an extra serving of Mistaken for Gay, happens to brothers Cyclops and Havok in an issue of X-Factor.
 * Batman had an example in the Golden Age, with the King of Cats, Catwoman's brother.

Fanfiction

 * In the Spirited Away fanfic Resilience of the Soul, Linca's Backstory involves the time she became upset when she saw her lover hugging a pregnant woman who turned out to actually be his sister..

Film

 * The plot catalyst in Gray Matters. The brother-and-sister team both kiss the same woman, too.
 * Star Wars VI: Return of the Jedi: Han briefly fears that Leia's avowed love for Luke puts him out of the picture, and promises not to "get in the way." But having discovered Luke is her brother, Leia is able to reassure him. (Of course, considering how vigorously she kissed Luke in the preceding film, Han's doubts weren't exactly unreasonable.)
 * A major plot point in Thoroughly Modern Millie.
 * In Zathura,
 * Used in The Thomas Crown Affair (the one with Pierce Brosnan), where.
 * Early on in With Six You Get Eggroll, Abby McClure and Jake Iverson, both widowed with children, make a date for Friday night. Jake calls to cancel, citing his daughter's birthday, so Abby goes to a disco with her sister and brother-in-law, where they see Jake dancing with a pretty young girl. Abby gives Jake the cold shoulder, but later learns that the pretty young girl is actually Jake's daughter when she sees her at her son's high school graduation.

Literature

 * This trope is a great staple of the literature of the 19th Century, as it allows the conflict and drama entailed by (usually) a woman flouting social norms, while still keeping the heroine innocent. Jerome K Jerome takes a moment to complain about theatre plays relying on this trope far too much.
 * In Susan Calvin's introductory story, a lying robot persuades her that she's made this mistake about a love interest.
 * The Tenant of Wildfell Hall by Anne Bronte. The hero, Gilbert, becomes suspicious of Helen's close relationship with her landlord Mr. Lawrence, and knocks the poor man unconscious in a fit of jealous rage. Of course, Mr Lawrence is Helen's brother. (As she's living under a false identity to evade pursuit by her alcoholic husband, they have kept their relationship secret, but their clandestine meetings have fueled gossip in the village.)
 * In The Dresden Files, this becomes a Running Gag.
 * In Blood Rites, between Thomas Raith and, made worse by the fact that 1) they cannot openly reveal their true relationship, and 2) they were sharing the latter's small apartment for an extended period after Blood Rites. And 3) Thomas was deliberately pretending to be gay in his public life for completely unrelated reasons. Considering all this together, we should have seen it coming from the paragraph after that "Wizard of Oz" poster on Thomas' wall.
 * In Dead Beat, Waldo Butters (after meeting Thomas for the first time),
 * In White Night, this is deliberately exploited to explain to building security why has a key to Thomas' apartment and has turned up there in the latter's absence.
 * Well, that and the unlikelihood of Harry being an NBA-sized gay burglar who works with a dog.
 * In The Count of Monte Cristo, Debray assumes Albert and Mercedes are having an affair after running into them at a seedy inn. The fact that Mercedes is veiled partly excuses the fact that he mistook his friend's mother for his mistress.
 * There's also an earlier scene where the Count sees a painting of a younger Mercedes in peasant dress and makes a comment about her being Albert's mistress. While he's only pretending to make that mistake and actually knows who Mercedes is, it's kind of weird that Albert keeps a picture that his mother had made for his father as a romantic gesture, and explains why he has such a massive Oedipus Complex in Gankutsuou.
 * North and South by Elizabeth Gaskell. Similarly, the heroine Margaret Hale's reputation is damaged in the hero's eyes after she is observed saying an intimate goodbye to a young man at a train station, and then lies about it. She cannot reveal that the young man was her brother as he is a wanted man.
 * In Bag of Bones by Stephen King, Mike learns that before his wife Jo's death she had been seen hanging around TR-90 with a man and being openly affectionate with him. He naturally assumes that his wife had been having an affair until he confesses his fears to Jo's brother - who is very quick to explain the truth.
 * The Wouldbegoods: This trope is cheerfully lampshaded. The child protagonists are reluctantly trying to reunite their adult friend with his long-lost sweetheart even though they fear this will mean he has less time for them. So they are pleased, rather than scandalised, to find the girl embracing another man. But their relief is short-lived and the narrator reflects, "He might have known it was her brother, because in rotten grown-up books if a girl kisses a man in a shrubbery that is not the man you think she's in love with; it always turns out to be a brother."
 * Sue Townsend's Adrian Mole, when aged fifteen, is allowed to witness the birth of his sister because the nurses assume he's actually the baby's father.
 * In one of Kathy Reichs' Temperance Brennan novels, several other characters mention seeing her boyfriend Ryan going out with a young woman. Its eventually revealed that she is his recently discovered daughter.
 * The Secret History: When Richard first sees Charles and Camilla together, he assumes they are boyfriend and girlfriend. He later learns that they are twins. Even later.
 * In David Eddings' The Belgariad, Ce'Nedra makes this mistake about Belgarion and his cousin. Her response to being corrected is, amusingly, equal parts "Whew, he's not attached" and "Shit, I'm in love with him!"
 * Averted in Northanger Abbey when the heroine sees her Love Interest with another woman on his arm and immediately comes to the correct conclusion that it's his sister, thereby depriving herself of a perfect opportunity for needless drama, as the narrator tells us.

Live Action TV

 * In Black Books, Fran gets annoyed when she sees her boyfriend having coffee with another girl, that turns out to be his sister. Partially lampshade hung when Bernard guesses, correctly, that that was the case before Fran actually reveals that to him.
 * Used secondhand in Bones where Booth and Brennan saw Sweets' girlfriend in a wedding dress hugging another man, and after some disagreement Brennan told him, made worse by her directly informing him that his girlfriend was engaged to someone else. Luckily for them when he confronted her about it she explained what had actually been happening (her trying on a wedding dress for her cousin, who's the same size), and they... reconciled.
 * Friends: Rachel is shocked that her boyfriend seems to be cheating on her, only to find that the woman he was hugging was his sister.

Sibling Ross and Monica Geller, who in spite of never having on-screen moments that may confuse people, are shocked to learn in the tenth season that they had actually kissed while teenagers (in fact, Ross was Monica's first kiss ever).
 * Grey's Anatomy introduced Patrick Dempsey's sister this way.
 * Two and A Half Men features brothers Charlie and Alan acting a lot like a married gay couple, which is a running gag in several episodes. For instance, in one occasion Alan's out camping, Charlie's asked by a friend about "the wife and kid".
 * Part of the solution in one House MD episode.
 * In Keeping Up Appearances, when Hyacinth sees Emmett, her neighbour Elizabeth's brother, for the first time, she believes her to be cheating on her absent husband. Although, it isn't because she sees him with her, it's because she sees him at her house, dressed only in a towel.
 * One episode of Power Rangers Lightspeed Rescue had Ms. Fairweather call a famous scientist named Clark to help her build some armour for the Rangers. They get along exceedingly well, and Clark is even given the honor of revealing her first name (Angela) to the audience. Meanwhile, Joel, who was in love with her, is seen getting very, very jealous, and even tries to sabotage one of their 'dates.' Finally, they get attacked by a Monster of the Week and, seeing Ms. Fairweather's obvious distress when Clark is still in danger, Joel decides he wants his beloved to be happy and saves his rival. Later, he gamely tells her how glad he is for her and that she deserves the best boyfriend ever. Naturally, this is when he is treated both to the reveal that he was her brother all along and to a kiss on the cheek. In a No Hugging, No Kissing series, even. Joel and Ms. Fairweather have the distinction of being the first PR Official Couple to get married.
 * Remington Steele: Steele once saw Laura talking with her brother-in-law and thought she was having an affair. It didn't really help that the brother-in-law kept kissing her on the cheek...
 * The brothers in Supernatural are frequently mistaken for a gay couple. This apparently does not stop being funny, ever. In this Crapsack World you take laughs when they come.
 * In the last season of Veronica Mars, Mac's newly introduced animal-rights-activist love interest turned out to have a female housemate, who triggered this trope.
 * The Vicar of Dibley: Done with Geraldine's love interest in the final episodes.
 * Firefly: When the box containing River is opened, Mal says scornfully, something to the effect of "Is she your lover, because you do seem awfully worried." Specifically, he accuses Simon of human trafficking, then sarcastically says something like "Oh, I'm sorry. Is this one for you, is it true love?"
 * Used in The Fresh Prince of Bel-Air, in which Hilary and Carlton were each flirting (separately) with an online admirer. They went to the restaurant to meet their date. . .only to discover that it was each other.
 * Full House had an episode where Kimmy and Stephanie see DJ's boyfriend Steve with a woman who turns out to be his cousin.
 * In Desperate Housewives, Lynette pretends to be a teenage girl in a chatroom so she can have real conversations again with her son. Unfortunately, he gets a crush on her online persona and she has to let him down easy with a goodbye message...which she signs "Love, Mom."
 * Inverted in Malcolm in the Middle, where Reese walks in on his wife cheating on him and she tries to pass off her lover as her brother. Having turned into an Extreme Doormat by then, Reese buys it, and even continues to call him her brother when he later walks in on the two of them in bed.
 * An episode of Frasier had Niles shopping for an engagement ring, accompanied by Frasier and Martin. Everyone in the store assumed the brothers were getting engaged to each other and applauded, much to their father's dismay. Seeing as how they do everything together, including going to their high school prom, this has probably happened to them a lot.
 * Played with a lot in Castle, where Rick says to Beckett more than once things like 'I've got a big date' or that he has a lady to see when he's actually just talking about his daughter Alexis. As a Genre Savvy writer, he does this deliberately to feed the Unresolved Sexual Tension and make Beckett jealous, which she does a poor job of hiding for someone trying to convince everyone (including herself) that her relationship with Rick is strictly professional.
 * In an episode of Drake and Josh, Josh sees his ex-girlfriend Mindy hanging out with and being affectionate with what he assumes is a new boyfriend. Of course, he turns out to be her cousin. When Josh asks why Mindy didn't say so in the first place, she says that Josh wasn't letting her explain.

Music

 * A darker variation occurs in The Tailor Shop on Enbizaka from Vocaloid. The tailor sees her lover with three different girls on different occasions, much to her annoyance.
 * This is the plot of "After the Ball."
 * The Gospel duo of Bebe and Ce Ce Winans could come across this way to people who don't know they're siblings. They have a very close relationship and mixed-sex duos who are married are more common than siblings.

Theater

 * In the Gilbert and Sullivan operetta Iolanthe, the twenty-five-year-old protagonist Strephon is caught embracing a young woman (actually his mother). Since his mother is an immortal fairy and, as such, appears to be about seventeen, Strephon's fiancee is understandably skeptical and promptly breaks off their engagement.

Web Comics
""And you're not sleeping with Dirk?" "Uh, no. I said England, not Georgia.""
 * Badly Drawn Kitties used this once. Lucy overheard Lydia's boyfriend Dirk and another woman declaring their love for each other, and assumed because of the woman's foreign accent that she wasn't related to Dirk. It turned out she was in fact his sister and she'd just been studying in England, and so had picked up the accent;


 * Given that Lucy had had a "unique relationship" with her own brother, that wasn't actually an unreasonable assumption on her part. But in the original continuity, Lucy and her brother were originally from Georgia (the US state), so there may have been some Lampshade Hanging in the rebuttal.


 * Ménage à 3: Zii makes this mistake, then, panicked, tries to cover herself under it. (Zii is new at monogamy.) Her boyfriend is sweetly understanding.
 * Done in Too Much Information, when the two main girls of Ace's Unwanted Harem sees him driving a pretty, blond girl around in his sports-car... it's actually his cousin, whom he's giving a ride to an abortion-clinic so her parents won't find out.
 * In Kevin and Kell, a well-intentioned Kell sets her friend Aby Eyeshine up on a blind date with Rudy's hunting coach, Sam Pardus. Sam and Aby, in accordance with the trope, are brother and sister, but Kell didn't know that (all parties found the whole thing amusing). Sam being disguised as a leopard (while Aby is a house cat) and changing his last name in order to coach the Caliban Academy team despite being domesticated makes it that much harder for Kell to find out.

Western Animation

 * Captain Planet: In the episode "Missing Linka", a disguised Wheeler helps saves the life of Linka in her hometown in Russia. He believes Mishka is Linka's old boyfriend, but in the end, like it turns Mishka is Linka's older brother.
 * In Garfield and Friends, "Wedding Bell Blues", Jon's cousin Marion, in town for her upcoming wedding, comes over to visit. She says the groom-to-be is named Harold John. When she points out the coincidence, and that, as a result of the wedding, she'll be "Marion John" (say it out loud), Garfield interprets the remark as the woman being Jon's secret fiancee and spends the episode worrying that Jon won't pay as much attention to him after the wedding, but more that their children will play harmful games with him. At the end, he discovers what the viewer knew from the beginning: that the woman is Jon's cousin and she is marrying a man whose last name is John. His attempt to disrupt the wedding is fortunately taken in good humor and the wedding continues.
 * My Little Pony Tales: In the episode "Sister of the Bride", Clover and her friends see Cheval (the groom) with another girl and accuse him of cheating on Meadowlark (the bride). Eventually, Cheval explains that the girl is his sister.
 * The Simpsons: Lisa has a crush on an older boy, then hears him having a phone conversation with a girl, saying that he can't wait for her to get there. When the girl arrives Lisa purposefully gets her lost in the woods, only to find out that her rival is, of course, the boy's sister. (The boy, naturally, does not take this well.)
 * On Phineas and Ferb, Candace becomes jealous when she finds her crush, Jeremy, "flirting" with a prettier girl who turns out to be his cousin.
 * In an episode of The Weekenders, Tish requests that she just hang out by herself for the upcoming weekend...and the others spy on her, suspecting ulterior motives, and see her hanging out with a boy her age all over town. When they confront her about her 'new friend,' she reveals first of all that it's her cousin Doug, and secondly, she had planned on being by herself that weekend, but Doug's visit was last-minute and her parents asked her to show him around.
 * Inverted on Daria—when Daria first starts dating Tom, Kevin and Brittany assume that he's her brother. It seems to be partially because they think it's weird for Daria to date, and partially because they know Tom was recently involved with Jane.

Real Life

 * Actor Samuel L. Jackson was once photographed going to dinner with a much younger woman, which caused some speculation amongst celebrity gossipers. This eventually reached Jackson's mother, who called her son to see if this was true. His response? "You crazy woman, don't you recognize your own granddaughter?!"
 * Ex-England football team coach Sven Goran Eriksson was snapped with a young blonde. the Daily Star "newspaper" ran a front page story, basically emphasising the fact that she was young, blonde and worth a few "Phwoar!"s. The girl turned out to be his daughter.
 * The "Kiss Cam" at any sporting event usually makes this mistake every once in a while.