Everyone Is Related

In some stories, it's not enough for the hero and villain to be long lost brothers, separated at birth. It's not enough for the protagonist to find out that the man he has chased down with intense hatred is also the man who conceived him, and thus they are linked by fate. Every main character is secretly related, and everyone they have even incidental contact with is a far removed cousin or uncle. If nothing else, The Dragon is secretly The Lancer's adoptive father, or the White Magician Girl went to the same monastery as the dread summoner who now threatens all of existence. For one reason or another, everyone of even marginal importance to the story secretly has some kind of connection to each other, to be revealed only at the moment of greatest dramatic impact (because the writers hadn't thought of it until just now). In particularly silly cases, you can gauge a character's rising importance in the story by the fact that they are suddenly revealed to have been related to someone else in the plot all along.

Very common in Soap Operas, which hinge on dramatic twists. You can't really blame them for eventually running out of reasonable ones and going back to the family well. Also common in particularly Mind Screwy and plot-heavy video games and long-running dramatic television series. Modern incarnations are often a Deconstruction, but not always. The presence of (fertile) immortals can easily lead to this, especially if the immortality is itself heritable.

Often Truth in Television when dealing with a feudal society. Those stories tend to revolve around royalty and nobility, who were intermarried so heavily that it's entirely plausible for every scheming duke and brave baron to be someone's uncle or cousin. This is possibly where the trope originates. Also true for humanity in general: recent studies indicate that the most recent common ancestor of all humans alive today may have lived as recently as three or four thousand years ago, while in most groups where immigration isn't the primary method of population growth, it is inevitable that this has happened much more recently. The way populations work, someone ends up either leaving a lot of descendants, or the family line dies out within a few generations. So if a person living now can trace their ancestry back to, say, Charlemagne, that means inevitably hundreds of thousands, and probably millions, of other people living today can do the same thing, which means they are all distant cousins.

Compare You ALL Share My Story and Tangled Family Tree.

Subtrope of One Degree of Separation.

Anime and Manga

 * Almost every single character from the Tenchi Muyo Ryo-Ohki! OVA is related in some vague way. Of the show's huge cast, there are probably only about three characters that aren't a part of that tree: Tenchi's two school-mates, and Doctor Clay.
 * Alright, Mahou Sensei Negima. I'm just going to put this whole thing in spoilers:  To be fair, not everyone is related, but at this rate, it won't take long.
 * Hayate the Combat Butler seems to have everyone related by convoluted means. Given that it's an Unwanted Harem..
 * Only Ayumu Nishizawa has escaped this fate so far. Either going to the same school or related... or both.
 * Ever try to make a Cardcaptor Sakura family tree? If they ain't related, they're dating...or they're related AND dating.
 * Similarily, try this with Tsubasa Reservoir Chronicle and it's various worlds and other plot twists - you'll end up with horrible migraine.
 * Rah Xephon has pretty much the entire case being related in some way or other.
 * And it makes things particularly confusing by having most of the characters be either lying or ignorant about their real relationships!
 * In Tenshi Ni Narumon.
 * Naruto has a lot:
 * In Umineko no Naku Koro Ni, every character is either a member of the Ushiromiya family, one of their maids/butlers, a witch, or a witch's furniture.

Comic Books

 * Due mainly to the Summers' Tangled Family Tree, the Marvel Universe sometimes looks like this. The time travel doesn't help -- Jean Summers (nee Gray) lost one baby to the future... who came back as two different superheroes from different futures!

Fan Fiction

 * Perhaps this should be it's own trope "Wold Newtonism" (http://www.pjfarmer.com/woldnewton/Pulp.htm ) For example, we know that Sherlock Holmes' family seat, Mycroft Hall, is in Norfolk. As all the county gentry are more-or-less related, we may assume that Sherlock and Mycroft were related to Lord Peter Wimsey. And we know that Sherlock & Mycroft had French relatives; perhaps BELGIAN ones? Perhaps that's why Hercule stayed in England after the war--he had English relatives?
 * Life and Love: Pokemon Journeys invokes this.

Film

 * Crazy Stupid Love: Sort of. The main character's friend meets a girl in a bar that made him want to be monogamous, and the woman the main character met in the bar.
 * An unusual variation: Kevin Smith's Askewniverse movies include a member of the Hicks family in every movie; in either a main, supporting, or cameo role. The character is always played by Brian O'Halloran.

Literature

 * JRR Tolkien's Middle-earth (The Lord of the Rings, etc.) has many related protagonists by virture of them mostly being from various prominent families or ruling dynasties. But there aren't really any cases of 'surprise-relatives' (which this trope seems to be about). Well, except perhaps Túrin getting told his wife is actually his grown-up and amnesiac baby sister, but the reader knows that from the start. This is most apparent in the First Age, where all the main Elves and Men are descended from the three 'fathers' of their respective races (Ingwe, Finwe and Elwe/Elu Thingol for the Elves, and Beor, Marach and Haldad for the Men), and there is much intermarriage between the descendents, which eventually leads to the birth of Elrond and Elros, who are descended from all of them simultanously. This trope becomes less apparent in the Second and Third Ages, as the First Age thinned the cast a bit.
 * Most of the wizarding families in Harry Potter are related in one way or another, especially through the Black family. One of Harry Potter's grandmothers was a Black. Ron Weasley likewise had a Black (a different Black) as a grandmother. Draco Malfoy's mother was Narcissa Black before she married. Likewise, one of Neville Longbottom's grandparents was a member of the Black family. And that's just the major characters...
 * Let's see, Tonks is the niece of Bellatrix Lestrange and Narcissa Malfoy, making her Draco's cousin. Well, just see the family tree yourself It's surprising that the Lovegood family isn't there.
 * Draco and Harry are actually distant relatives, who share a great-great-grandfather. Even one minor character in Slytherin, who is not much more than a sparring partner for Hermione in the second book, is related to Harry and Draco. Her great-great-grandmother is their great-great-grandfather's wife. And of course, by the end of the story
 * According to the book of Genesis, every human being that ever lived is descended from Adam and Eve, and everyone born since the great flood is descended from Noah. One might conclude at first that this means that any kind of sex is incest, except for two things: First, the gene pool was still clean at the time of the flood, and the incest taboo wasn't needed until centuries later when God handed the law down to Moses. And even then, God never prohibited first cousin marriage.
 * Heavily subverted by Stationery Voyagers: Other than a few marriages resulting later on, almost NOBODY (important) is blood-related (or for that matter, Ink-related) to anyone else (important)! Yet, while blood relations are severely lacking among major cast members, everyone is "connected" in more abstract ways. (See that work's official page for an in-depth analysis.)
 * In any book after the first in any Piers Anthony series, EVERY CHARACTER EVER. Even, like, the robots and stuff. He'll find a way.
 * The Incarnations in Incarnations of Immortality (see One Degree of Separation for explanation of this tangle).
 * Let's not forget Dune, folks. When Jessica and Paul are on the run from the Harkonnens, Paul drops a bombshell: "We're Harkonnens."
 * The main premise of The 39 Clues book series is groups of people, all members of the vast Cahill family, vying to find the secret of the family's fame and power.
 * In the Outlander series, characters are revealed to be related even though they were born two centuries apart.
 * In the Warrior Cats series, considering that each clan has its own Tangled Family Tree, and that cats inside one clan are not allowed to mate with cats from other clans (though some still do), it is not surprising that after a few generations of mating with each other, almost everyone is related to one another inside any given clan, except for newcomers.
 * In Robert A. Heinlein's Time Enough for Love, protagonist Lazarus Long is the oldest living human being, having survived over 2,300 years thanks to an abnormally long natural lifespan and the technology of human rejuvenation. It is statistically estimated that, if you claim any ancestry at all from the Howard Families (humans who took part in a breeding program designed to improve longevity), there's an 80+ percent chance that you're his descendant, and the novel makes something of a Running Gag about everyone Lazarus meets telling him how closely related they are (fifth generation descendent of his eight wife, etc.). This even extends outside the Families, as he's had uncounted unregistered children over the centuries and at one point relates a story about meeting a pair of slave twins whom he suspects of being his great-to-the-nth grandchildren.
 * The medieval epic poem Parzival by Wolfram von Eschenbach lives this trope. The titular hero Parzival turns out to be closely or distantly related to just about every person he meets in the course of his adventures. This is generally interpreted as Wolfram telling his readers that all of mankind is one big family, even across national and religious divides - Parzival has an elder half-brother, Feirefis, who is a Muslim (although at the end of the story he becomes a Christian) and has black-and-white piebald skin, being the son of a white father and a black mother. For the time of the Crusades, when it was written, this was a quite unusual message of tolerance.

Live Action TV

 * Heroes is another example. With each season, more familial bonds spring up between main characters that originally lived on completely different continents.
 * Then again, powers are genetic, so it's a convenient way to introduce a new character with powers by saying they're a relative.
 * As mentioned in the trope, most Soap Operas tend toward this eventually. If nothing else, it's an inevitable consequence of the constant marriage and divorce of everyone in the show.
 * Lost has had a few. Jack and Claire  We had met Widmore, Eloise, and Daniel in seasons 2, 3, and 4, respectively, but it wasn't till season 5 that we found out Widmore and Eloise
 * Which makes Penny, and Desmond and Daniel- who were already good friends-
 * Hilariously enough,
 * Interesting variation:  comes out as a shocking twist, but unlike the above examples they were perfectly aware of their relation and simply chose not to tell anybody.
 * The Pretender where Jarod has an evil long lost older brother Kyle and a sister Emily. His arch nemesis Ms. Parker has an evil long lost twin brother who was separated at birth Mr. Lyle. Ms. Parker and Mr. Lyle's mother Catherine has a son named Ethan with Jarod, Kyle and Emily's father Charles. Mr. Parker the supposed father of Ms. Parker and Mr. Lyle turns out to be sterile and thus not their father. Their actual father is Mr. Raines who is the long lost brother of Mr. Parker but who was raised in foster care. Mr. Raines and his wife Edna also have a daughter named Annie. Mr. Lyle and his supposed father (but actually his uncle) have both slept with Brigitte and it isn't clear who her baby's daddy is. Oh and Jarod also has a clone.
 * Most of the characters in It's Always Sunny In Philadelphia. Frank originally thought was the father of twins Dennis and Dee. Turns out their mother Barbara had also been sleeping with another guy named Bruce but she dumped him because she thought Bruce was poor (he wasn't). Charlie discovers that Frank may be his biological father when Frank reconnects with Charlie's mother Bonnie via Facebook and Charlie who doesn't know who his dad is realizes Frank and Bonnie slept together roughly 30 years ago. Frank swears that Bonnie was a tramp and aborted the baby but Bonnie swears that Charlie survived the abortion. Later in a scheme to get back Frank's money that Barbara left Bruce upon her death Frank and Dee (his legal daughter) pretend to be engaged in an elaborate scheme to trick Bruce out of the money...they even go so far as to marry. Frank later also has a same sex marriage with roommate (and possible son) Charlie. Dee becomes the mistress of Bill Ponderossa...the brother of her twin brother's wife. Mac isn't really related to any of the characters but he has slept with Dennis mom. Also Mac's ex-girlfriend Carmen the tranny is the biological father of the baby Dee gives birth to.

Mythology

 * Arthurian legend also tends toward this, for the same reason that most feudal stories do.
 * Greek mythology tends to include complex genealogies for almost every God, Goddess, monster, king, hero and villain mentioned. Mostly connected to each other. For example, Theseus and most of his early foes were all said to be sons of Poseidon. Penelope was a paternal first cousin of Helen and Clytaemnestra. Making their respective hasbands Odysseus, Menelaus and Agamemnon kinsmen. And so on. The few figures not related to anybody else tend to be either insignificant or our knowledge of their legend is mostly fragmentary.
 * Agamemnon and Menelaos were brothers, and great-grandchildren of Zeus, who was the brother of Poseidon. Oh, and Helen was the daughter of Zeus, so a not all-that-distant blood relative of her husband and a first cousin of Theseus.

Video Games

 * On Tekken most of the main characters are Mishima's. Jin is Kazuya's son, who is Hehachi's son who is Jinpachi's son. And Jin's mom turns out to be Asuka's aunt.
 * Final Fantasy VIII springs to mind almost immediately. Three quarters of the way through the game it turns out that many of the characters lived in the same orphanage as children and then forgot about it; one of the side effects of the Applied Phlebotinum the game's magic system revolves around happens to be memory loss. And Edea, who appears to be the Big Bad but is in fact just the Big Bad's mind-controlled Dragon happens to have run said orphanage, and is the wife of the headmaster of Balamb Garden, the military school most of the main cast attends in the present day...which was established (by Edea) to fight Sorceresses like Edea.
 * To make it a lot more (relatively speaking) plausible, many of the mercenaries from Gardens were recruited from the said orphanage. The one character who didn't have any earlier ties with the rest of the cast isn't from a Garden. That being said, her mother did know and date the main character's father... Thankfully (her being the Love Interest) the possibility they are in fact half-siblings as result is ruled out.
 * In Haunting Ground
 * In No More Heroes, almost every character important to the plot ends up being related to . Don't think about it too hard
 * In the normal ending of Chrono Trigger, it's revealed that . And another, far less serious ending reveals that   -- though it's implied this is only true for this particular ending/timeline.
 * 12 out of the 18 characters Umineko no Naku Koro ni starts out with are direct family of each other whether by blood or by marriage. Then again, most of the story takes place during a family conference. (The other 6 characters are five of the family head's servants, and a friend of his.) As for the characters who show up later (and there are a lot), a number of them are Meta characters, two characters are based on characters from a previous work,
 * Basically every present-day character in Fate/stay night, although you can somewhat include many of the Servants as well. Where to begin. Shirou is schoolmates with Rin, Sakura, and Shinji. Shinji's younger sister is Sakura, .  . Kirei was Rin's father's apprentice,  .  . Shirou's only friends at school are Issei and Shinji, who both happen to be the school's cool kids (as a side note, Issei has an old rivalry with Rin and Shinji has a crush on her), while Shinji   and Issei lives with and is soul brothers with  . Shirou's Servant, Saber, , during which time  ,  . Shirou was formerly in the Archery club with Shinji, Sakura, and Ayako, and Ayako happens to be Rin's old, close friend. Taiga is simultaneously Shirou's neighbor, childhood friend, guardian, homeroom teacher, and former club adviser, and her grandfather is an old acquaintance of Kiritsugu who lets Shirou do his bike maintenance. And I shouldn't even need to mention  , should I?
 * And that's just the characters important enough to get face time.
 * And ignoring the relations to characters in Nasu's other works.
 * Infinity series
 * In Ever 17 But  is just some
 * 12Riven"
 * Kingdom Hearts gets points for doing this in a series without any Non-Disney relatives. There are a total of five characters connected to Sora alone.
 * And if you put the pieces together after 358/2 Days and Birth Before Sleep, then (deep breath)
 * The Fire Emblem series is notorious for this, but the most notable one is Genealogy of the Holy War where thanks to the "second generation" mechanic, with proper breeding the entire army really can be composed of various siblings and cousins and cousins' cousins, etc.
 * Which can be paired together.
 * Apollo Justice: Ace Attorney really liked this trope.
 * Oh, and don't let us get started on
 * Odin Sphere.  Part of the fun in Odin Sphere is figuring out the ridiculous family trees.
 * Metal Gear. METAL GEAR. It is made of this trope. It started out reasonable, but as time went on and prequels began introducing various characters's parents and so on, it quickly turned into a very matted family tree. In most cases the relationships are adoptive and the romantic relationships are non-incestuous about seventy percent of the time, so it mostly just leads to a lot of Fridge Horror for Shippers who work out that, say, Big Boss and Ocelot both have the same mother (spiritual for Big Boss, biological for Ocelot), that Solid Snake is technically Raiden's uncle and Gray Fox's brother, that Otacon's mother had been in love with and implicitly bonked Solid Snake's 'grandmother', or that shipping Big Boss/Roy Campbell projects a pretty skeevy dimension onto the canon pairing of Solid Snake/Meryl.
 * In Kara no Shoujo it doesn't seem this way at first, but once you have the full story it becomes clear how much this applies. This is not taking into account the stuff you know from the start like how everyone went to school together or still goes to school together and whatnot.

Web Comics

 * Homestuck: When it's not by blood, it's by adoption.
 * In Kevin and Kell, many of the characters marry into the Dewclaw or Kindle families, thus becoming related to the main characters, or are revealed to be related to them somehow (Corrie turns out to be Ralph's daughter, and a minor character, Wendell, is one of Dorothy's over 240 grandchildren). Some of the fans have even come up with a name for the phenomenon, "Dewclaw assimilation".

Web Original

 * The Gungan Council calls its family trees a "Family Wreath." This is usually due to massive families like the Draclau, Marzullos, and Decuirs having new family members every other day, and then those members marrying non-family members. It doesn't take much effort to be related to every single character on the site by just marrying one person.

Real Life

 * Common in Iceland, which has a small, almost completely genetically homogeneous population that has lived in geographic isolation for over a millennium. It's not unheard of to start dating an erstwhile complete stranger, and then encounter them at a reunion of extended family. This high level of national endogamy makes Iceland very interesting to genetic researchers.
 * The human species as a whole has relatively little genetic variation, as the result of a "bottleneck" where the population decreased to very low numbers about 70,000 years ago. In fact, all living humans are descended from "Adam" who lived about 60,000 to 90,000 years ago, meaning that we are all related, albeit distantly in most cases.