So you've got two healthy unrelated teenagers whose parents have recently gotten hitched. Said teenagers happen to be of the opposite sex. Whether they find it squicky or not, hormones are going to fly.
Usually this is seen in the undertones of their behaviour in a show, and it almost always happens when two people become step-siblings in adolescence. Sometimes though, this is unintentional on the part of the writers and attempts to make new step-siblings act like a long-time brother and sister just come off as flirty.
The Romance Novel is also fond of this trope, generally involving older stepsiblings. It has the double effect of giving them reason to resent each other over their lives' disruption and giving them no way to escape the other entirely (without cutting off from their parent as well).
- The manga Yoru Made Matenai is built around this trope, with a wee bit of borderline Shotacon fanservice.
- The main couple of Marmalade Boy, which starts with the two leads's parents switching partners and then all deciding to live together in one Big Fancy House.
- The whole point of Akuma de Sourou. Kayano and Takeru are in high school when their parents get engaged and start living together. Takeru heavily pursues Kayano, and naturally she gives in. Things get... complicated... as Kayano tries to keep their relationship secret to avoid breaking up their parents'.
- Though not teen-aged, Erio and Caro from Magical Girl Lyrical Nanoha StrikerS were both technically adopted by Fate and, as shown in the companion manga, think of themselves as step-siblings. However, their relationship is so close as to be romantic that even other people in-universe tried to push them toward each other. Hell, as mentioned in the Official Couple page, by the classic definition of trope—both being overtly paired off and it seems understood by the cast that the two are an item—they pretty much are one.
- Actually pretty much does become official as of Force.
- This is the starting point of an h-manga titled Giri Giri Sisters: Mion has fallen in love with her classmate Tsukasa, but before she gets up the nerve to confess, her widowed or divorced mother marries his widowed or divorced father. Compounding the problem, her younger but much curvier sister Kotone (Mion also suffers from A-Cup Angst, especially by comparison to Kotone) is likewise hot for their new stepbrother, and not at all shy about saying so and demonstrating it. The good news is, Kotone's not just willing but eager to share Tsukasa with her big sister ... and Tsukasa describes having sex with Mion after Kotone as "saving the best for last"....
- Kiss×Sis is about two twin sisters who love their stepbrother very much. Their mom and the guy's dad approve. Only the guy doesn't. Hilarity Ensues.
- Another rated manga example, Ai Don't Koi from Yasuhira Nakanishi. The main character struggles between two choices: one is his Unlucky Childhood Friend whom he always thinks of as a little sister, another is the girl whom he had a gigantic crush on - until her mother married his father.
- Yuka and Atsuro Kiryuu in Sensual Phrase, though only in the manga. One of the side-stories focuses on this, and specially when Riri Kinoshita, an Idol Singer who has a crush on Atsuro, blackmails him into pretending to be her boyfriend under threats of spilling out the beans about their "incestuous" bond, which would ruin Atsuro's budding career.
- Cherry Juice: As the series progresses, step-siblings Minami and Otome often switch between sharing flirtatious romantic moments and Belligerent Sexual Tension.
- Subverted in a case of Detective Conan. A woman named Kimie had to give her toddler daughter Yukiko away to her husband's family due to the terrible circumstances she was going through after her husband was in jail, and didn't know what had happened to her for many years. In the meantime, she got married to a man named Tsujimura, who had just gotten divorced and had a son named Takayoshi. Several years later, Takayoshi falls in love with a girl and brings her photo to his dad and stepmom... and the girl turns out to be Yukiko, who doesn't know anything about it. Tsujimura has a Freak-Out and forbids Takayoshi from dating Yukiko, having recognized the girl's identity as his wife's estranged child... which then leads to a horrifying reveal. And then It Gets Worse.
- This is the entire premise of Kiss×Sis, with teenaged Keita pursued vigorously by his older twin stepsisters Ako and Riko.
- Cruel Intentions ramps this trope Up to Eleven, with the plot driven by a girl promising her besotted stepbrother sex if he does what she wants. Not Another Teen Movie parodies this with them actually being related, and the brother being utterly repulsed at the idea, but going along with The Bet for other reasons.
- Clueless, of the Belligerent Sexual Tension variety.
- A Very Brady Sequel approaches this trope when Greg and Marcia are reminded that they actually aren't at all related.
- Drive Me Crazy has a variant of this -- the two teenagers have a mostly-normal romance, and their parents get married at the end of the movie.
- The horror movie Teeth. The brother has spent most of his life with a crush on his step-sister and despises their parents for making them siblings.
- The comedy Lone Star State of Mind (produced by Edward Zwick, of Glory and My So-Called Life fame) has stepsiblings who are engaged. They had been dating when their single parents married each other and continued the relationship after that.
- This is brought up On a Clear Day You Can See Forever. Todd Pringle (played by Jack Nicholson) points out that, since he is just as much in love with Daisy (played by Barbra Streisand) as Doctor Chabot is, perhaps he (Todd) might make a suitable match for her. When Dr. Chabot points out that Todd is Daisy's brother, Todd responds with, "Only by marriage... she's my step-sister, not my blood-sister."
- Blair and Aaron in the Gossip Girl books.
- In Twilight all the Cullens are
stepadopted siblings and most of them are in a relationship with another Cullen.
- Though this is just what they play for everyone else to explain why they move and cohabit together—none of them are actually "blood" relations, and the romantic and sexual relationships started before or at the same time as the masquerade.
- Julia and Tiberius in Jeg elsket Tiberius (I Loved Tiberius).
- Harry and Elaine from Dresden Files. They were both adopted by their Evil Mentor in their early adolescence. Once puberty struck they took their relationship in the expected direction.
- Big Trouble by Dave Barry subverts this. Matt and Jenny have a strong crush throughout most of the book. When their respective dad and mom hook up in the epilogue, however, they only go out on a few dates before realizing the whole thing's a little weird and deciding to just be friends.
- In the Amelia Peabody books, Ramses and Nefret were raised as siblings from the ages of ten and twelve, respectively, but ended up falling in love and marrying, albeit with some complications.
- Malevil has Jacquet and Miette. They are lovers as well, but Jacquet has to share her with every other man at Malevil.
- First, Alanna and George were involved. Then they weren't. Then Alanna's adoptive father Myles and George's mom Eleni fell for each other. Then Myles and Eleni got married. Then Alanna and George Hooked Up Afterwards. So, yeah. Sort of.
- Even though the boy comes into the family through adoption rather than marriage, Wuthering Heights fits the rest of the trope, and is what happens when this sort of relationship goes spectacularly badly wrong. (Although there is a school of thought that believes that Catherine and Heathcliff are actually half-siblings.)
- Drake and Josh Between the two stepbrothers, surprisingly. They've even kissed!
- Life with Derek: This seems to be unintentional on the writers' part, but very intentional on the actors'.
- Lost: Boone and Shannon seem to hate each other, but it's stated that Boone is in love with her and they have sex during a flashback.
- The Secret Life of the American Teenager
- Reaper has Sock getting a stepsister. She thinks of them Like Brother and Sister, but tends toward much I Didn't Mean to Turn You On. He's trying to get in her pants. (He eventually succeeds.)
- Los Serrano has the relationship in between Marcos and Eva. Lampshaded in a dream sequence ( despite the series itself being All Just a Dream, but anyway) where Marcos says their relationship isn't incest, "more like stepincest".
- From the Taiwanese drama Devil Beside You, Qi Yue was being bullied by the son of the school's president, Jiang Meng, only to fall in love with him, after realizing that he was a Jerk with a Heart of Gold and actually a decent guy. Before either of them could understand their feelings, her widowed mother and his divorced father began dating, eventually becoming engaged. One of the main conflicts of the story is the main couple trying to work out their relationship amidst their parents' relationship as well.
- Happened in the regular The Brady Bunch too. Greg and Marsha have to share a room in the attic with only a bedsheet seperating the room. Greg (and the audience) watches her silhouette change behind the sheet, and the UST is on.
- Morgana and Arthur in Merlin had a very charged relationship in Season 1.
- "The Crystal Cave" reveals that Arthur's father Uther is Morgana's biological father, having had an affair with Gorlois' wife Vivienne.
- Glee: Now that their parents are married, Kurt and Finn. It certainly didn't help that the title of the episode was "Furt". Finn even sang a song for him! Of course, Kurt originally tried to get Burt and Carole together in an attempt to invoke this, as he had a crush on Finn for months beforehand (it didn't go very well).
- In every episode thereafter, however, well and truly averted; Kurt's interactions with Finn are nothing at all like his flirty, adoring interactions with his boyfriend Blaine, and the two stepbrothers seem to be uninterested in each other much of the time, and even vaguely dismissive. It's unsurprising given that Kurt got a double dose of Finn's real personality in 1x20 and 2x04, and that would be enough to kill anyone's interest. Except Rachel's, of course.
- The Fresh Prince of Bel-Air: Will's mother and Lisa's father take a liking to each other, and after their kids decide (at the altar) that they're not ready to be married, their parents step up, saying "Why waste a perfectly good ceremony?"
- The German comedy series Türkisch für Anfänger ("Turkish for Beginners", 2006-2008) is about the family consisting of therapist Doris Schneider, her children Lena and Nils, her lover, later husband, police inspector Metin Öztürk, and his children Cem and Yagmur. One of the major plotlines of the series is the on-again-off-again romance between stepsiblings Lena and Cem. At the end of the third season the two are a couple and Lena is pregnant.
- On Degrassi, Clare dates her mother's fiancee's son Jake. Later on, when they are thinking about having sex, Jake says they should just end the relationship because if they break-up painfully it would get awkward, seeing as they are now related.
- The very last strip of Ozy and Millie, with a few hints shortly prior, indicates this is going to be how things are for the title characters.
- Valkyria Chronicles: The main character makes his "Adopted" sister's non-blood relation clear to a very suspect degree.
- Master Li in Jade Empire runs a martial arts school, and his two prize students (Dawn Star and the Spirit Monk) both are his adopted children. After certain revelations about Master Li, it becomes apparent that there is also a case of flirty stepcousins if the Spirit Monk enters into a relationship with Silk Fox.
- 6teen has Jonesy and Jen in spades; they even kiss and Jen admits she used to have a crush on him.
- Martin Mystery: It takes the subtext to the nth degree. They were really pushing it to the limit. That's what happens when you take characters who were lovers in the original comic and turn them into Not Blood Siblings.
- In one episode of The Simpsons, Homer and Marge get squicked out by the thought of Homer's dad marrying Marge's mom, since it would lead to this.
Homer: If your mom marries my dad, that makes us brother and sister! And think of the kids: they'll be horrible freaks with pink skin, no overbite, and FIVE FINGERS ON EACH HAND!!