All Assorted Animorphs AUs: Played for Laughs in "What if Peter and Naomi got married?". The premise already makes Marco related to two of the other Animorphs — he and Rachel are stepsiblings, which also makes him Jake's step-cousin — then at the end, Eva (Marco's biological mother) gets together with Loren (Tobias' mother, Ax's sister-in-law), which makes all the Animorphs technically related except Cassie.
Ashes of the Past: Ash's Ivysaur claims this is the reason he knows such a large and varied number of moves.
"I know of a Charmeleon, a Sableye and a Skarmory in there somewhere. It's more of a family Kudzu..."
Bakura magically fathers a child with an unnamed yuki-onna. This is Yukito.
Yukito, who is also a Gender Benderincubus/succubus, becomes mother to a child, Ryou, who is fathered by a human... who is possessed by a ghost. They say that technically Ryou's father is the ghost in a human body.
Yukito later has triplets, with three different fathers — one from his own boyfriend Jounouchi (who turns out to be a long lost descendant of vampire Kenshin; one from Kenshin, who is the head of his own vampire/youkai clan; and one from both Kenshin and Saitou.
Bakura "marries" into the Kaiba family, thus bringing in that small clan, who are actually already related to Saitou.
To top it all off, due to some magic Yuki's fifth child, Yue, is actually both Bakura and Ryou's son.
Pretty much all the main characters of the Blooming Moon Chronicles are related in some way, whether it is by blood, marriage, or adoption.
Deceptive Reality probably takes the cake for shocking Luke, I Am Your Father plot twists—Snape is revealed to be Harry's father but it gets even more complex with the revelations that Snape is actually a Potter by birth and was James' twin brother but he was blood-adopted out to the Potter's family friend Eileen Prince and her husband. Also Lily isn't a muggleborn but she was adopted out to her squib relatives because her parents feared she would be targeted seeing as she is the daughter of McGonagall and Voldemort.
In Déjà vu no Jutsu, orphan shinobi are considered the ward of their jounin sensei. This makes Jiraya Minato and Natsumi's 'father,' although she sees him more like an uncle, and the Sandaime Hokage their 'sort of' grandfather, since Jiraya was an orphan while learning under Hiruzen. Since Natsumi was also Hatake Sakumo's kenjutsu apprentice, this makes her Eri's daughter, and Kakashi and Midori's sister. We also have all the honorary relationships between the clans that practically make them family. There is a complete explanation in story.
In the The Loud House fanfic The Edgy House (as covered in Peeking Through the Fourth Wall's fourth After Dark episode), the Loud family tree is depicted as a snarled mess thanks to adultery and incest — both parental and sibling — among the members. Lily, for example, is the result of Parental Incest between Rita and her son Lincoln, who is himself the child of one of Rita's several affairs rather than her husband, Lynn Sr.. Subverted as it's later revealed to be Lincoln's version of The Aristocrats joke.
Several of the Original Characters introduced in the Freedom City fan expansion "World of Freedom 2.5" turn out to be related, and the bios generally only tell you what the characters themselves know, which is generally very little, so it all needs to be pieced together. However, it appears that the Extremist is the daughter of Miss Murder and Mark Hazard, which makes her the half-sister of Kent Hazard, who fathered a child on Ebony Sin (which ties the family into many canon Freedomverse characters, including the Raven dynasty), who became Trouble/Concrete Angel. As of the beginning of the story "The Gathering", all surviving characters are in one place, Kent and Trouble have formed a close connection, and none of them know they're related. (Also, Kent rescued Trouble many years earlier, but neither of them has made that connection either.)
Due to adoption and the time travel effects of Instrumentality in Ghosts of Evangelion, Misato ends up simultaneously being Asuka's mother, mother-in-law, and stepsister.
Incorrect Smash Bros Quotes has the family tree created as a result of the Palutena Party (a polyamorous relationship between Palutena, Ganondorf, Bayonetta, and Dracula). Let's see... Palutena and Dracula sired Sothis (making them Seiros/Rhea's grandparents) and the latter has a kid from another relationship (Alucard), Ganondorf sired Captain Falcon with Palutena, Joker with Bayonetta, Hapi with an unknown woman, and adopted Chara as his heir, and that's without getting into if you consider Pit (and possibly Dark Pit, by extension) Palutena's adopted son.
In general, the Smashers' family tree is a convoluted mess of Crack Pairings, Crossover familial ties, and interdimensional adoption. It's so bad that an attempt to actually make a family tree drove Galeem completely insane.
In The Institute Saga, Clark gets teased by Alex Masters over whether his adopted sister Rogue will remain so or become his daughter due to her once being Mystique's adopted daughter. Of course, he then claims to be glad that his family tree isn't as tangled — when it's even more convoluted.
Jonathan Joestar, The First JoJo: Jotaro lampshades this regarding his own relationship to Josuke and Giorno, the former due to being his uncle in spite of the age gap, and the latter due to the overall generational gap in being Jonathan's son.
Jotaro: For fuck's sakes, he's my damn uncle? In what world is an uncle younger than his nephew?
Justice League of the Rebellion: Lelouch and Nunnally vi Britannia already had this with their father Charles being the Emperor of Britannia and the supervillain Brainwave while their mother Marianne is the human form of Eclipso. However, as the adoptive children of Mister Miracle and Big Barda, they also have ties to both New Genesis and Apokolips, with both Highfather and Darkseid as their adoptive paternal grandfathers.
Charles's children either start out or become Composite Characters with different DC Comics characters. Lelouch and Nunnally are Batman and Oracle respectively, Cornelia and Euphemia join the Justice League as Hawk and Dove, Schneizel starts off as Owlman and then becomes the Joker, Oddysseus gets turned into the story's Two-Face, Clovis undergoes Sanity Slippage and becomes Firefly, and Guinevere becomes Poison Ivy.
Khaos Omega has implemented this in a few cases, usually in the form of massive harems caused by the acquisition of a particularly devastating superweapon called the Rainbow Omega Cannon. Self-based OC Jet Brazie is the first to get this when Galaxy Angel II's Anise Azeat gains the weapon.
Then Anise, along with three other high-ranking Rainbow Angelsnote the name Jet's harem adopted in 2039 (namely Anise's homeworld teammates Lily and Natsume, plus unrelated-outside-of-the-harem Kandyce), become the leaders of lesbian harems that serve as sub-divisions of the Rainbow Angels. Anise's harem, the Amethysts, extends the sub-division chain two levels further with the Echo Hornets, co-led by Amethyst members Fontina Evans and Michaela Martin, which has its own sub-division for their realm-jumping agent Io Martinez.
Further complicating matters is the revelation that Jet's eldest sister Hollie can only develop romantic feelings for married women; the sheer size of Jet's harem gives her a convenient group to prioritize first. Among the Rainbow Angels she's gone all the way to official marriage with are Anise (Hollie replicated the spell that officially married Anise to Jet), Lily (Hollie also picked up a delayed-activation variant that didn't deploy until 2304), and Kandyce (Hollie had nothing to do with this 2197 delayed-activation case, as Violetta Narimiya is the actual caster here; Violetta chose Hollie so Kandyce could have her dream bride-with-male-bride wedding with Jet). When Jet learned of this condition of Hollie's, his first reaction was to ask Hollie's BFF Maeve (one of the rare few Rainbow Angels to give up her maiden name entirely) why she hadn't added herself to the equation.
Light is L's father, which means Light and Creepy Dude (who had been established as L's father) are the same person.
Dark's girlfriend/mother Samanther is L's mother as well, so Light is both L's father and half-brother. This also means Samanther is The Girl from the Bus.
Since Watari is Creepy Dude's father, Watari is Dark and Soichiro. However, Near and Watari are clones and switch places at one point, so Watari is also Near.
L is also Watari.
Light ends up becoming Blud.
Light also somehow becomes Might Yagami, a dog, as well.
So Dark, Soichiro, Watari, L and Near are the same person. Light, Creepy Dude, Might and Blud are also the same person, as are Samanther, Light's mom and The Girl from the Bus. And since two of them are the third one's parents, this means every important character who isn't a clone is related by bloodnote and this is ignoring the fact that Dark died and possessed Light's body at one point, essentially killing him. Even the author seems to forget this.
The Lion King fandom practically revolves around this trope. There are full-on wars just on who Nala and Kovu's fathers are. Plus, there's a full series of books that fill in some of the gaps and leave more for fans to speculate on, but just try stuffing them all in along with the Outsiders and you get somethingthat looks◊like these.Warning: The last one may result in an Archive Binge.
In No Featherbed For Me, Arya Stark escapes marriage to Renly Baratheon to run away with Aegon Targaryen, with whom she has three children. After Aegon takes the Iron Throne, he sets Arya aside to marry a Tyrell and has children with her. Meanwhile, Arya has an affair with Gendry Baratheon, who is married to Sansa, and they have a son. Then Aegon remarries Arya while his Tyrell wife is still alive, because Targaryens, and they have two more kids. Arya's older son then marries both his sisters, because Targaryens, but one of his wives secretly gets pregnant by her half-brother (Arya and Gendry's kid) because that's less weird.
In A Potted Black Rose one Rosa Rosier had three daughters. The eldest, Olivia, was a squib who married Muggle Henry Evans and gave birth to squib Petunia Evans. The middle daughter, Druella, was mother to Narcissa, Andromeda, and Bellatrix Black, while the youngest, Lillian, was Harry's mother.
The RWBY Loops gives us the Extended Remnant Looper Family Tree, which spans nine universes and has between forty and fifty members. As noted above, it has its own page.
The So Bad, It's GoodSonic the Hedgehog fanfic SONIC HIGH SCHOOL has Knuckles the Echidna be reincarnated as Rouge the Bat and Espio the Chameleon's son due to some magic in Chapter 10. Thus, he is simultaneously Knuckles Jr.'s father and half-brother.
When James and Lily in Scion of Sorcery got into a fight, Lily wound up in a one-night stand with Stephen Strange. James and Lily got back together and they had Harry, James accepting him as his own son and even going through an adoption ritual, making Harry James's heir magically as far as anybody is concerned. Because of all of this, Harry has the legal rights, magical power, and perks of Houses Potter, Evans and Strange as their heir.
Teenage Mutant Ninja Turtles: From the Hidden City: The titular characters' immediate family is... convoluted, to put it mildly. Splinter is their biological father, as the source of their human DNA. However, because they were adopted by Splinter's brother, they're considered his nephews and not his sons. Said brother, Saki, had another child, Karai; who is simultaneously the turtles' adoptive sister and their biological cousin. There also turns out to be a fifth turtle, Artemisia. She's their biological sister, but she was raised by Draxum; who is married to Splinter and thus technically the turtles' uncle, which also makes Zia their cousin. Trying to figure out whether or not that makes Splinter the stepfather of his own daughteris liable to cause spontaneous cranial detonation syndrome. Even the author has joked about how crazy their family tree is.
The Harry Potter example (see the Literature page) is implied in That Malfoy Family (a Harry/LuciusMpregfanfic), in which Harry thinks it's very crazy that he's carrying Draco's sibling when they're the same age.
Harry: This will be the weirdest family. Draco: If you think that, you haven't seen the Malfoy family tree. [shuddering] That really isn't a pretty sight.
In the Transformers fanfic Things We Don't Tell Humans, Cybertronian families are generally huge and complicated. Not only is adoption common among the main characters, it is also possible to independently adopt someone as a sibling without them being necessarily considered related to anyone else in your family (except any other siblings you share a brother-bond with).
A series of short Vampire Knight fanfics (Irit's Daughter, Gertrude's Closet, Jocasta at the Crossroads, and Vashti's Feast) written by Ms. Izzy takes the already messed up Kuran family and tangles it further. The fact that Yuuki's parents are canonically brother and sister seems tame compared to Ms. Izzy's version. While the family itself is pretty small, it is impressively tangled, with their unnamed mother being the product of another brother and sister pair, and upon her mother's death took it upon herself the continue the family line with her own father, giving birth to Rido and Haruka with him. It is further implied that she had conceived Juri with her brother/son Rido, rather than her father/husband. Not bad for a series that's under 2300 words.
Voltalia gives us the Voltalian Cartoon Universe Family Tree. This family tree certainly is tangled AND huge. And that's just a work-in-progress. Rumor has it there's more she's added already.
In A Student Out of Time, not only do we have the parents of Class 77-B, but Hajime and his partners get a glimpse of the future where they learn they have eight children, seven biological and Kotoko, who is adopted by Mikan on October 13th, 2012. In addition, as a consequence of saving Natsumi Kuzuryu, Word of God confirms that she later has a son named Haruto, who goes on to marry Hajime and Hibiki's daughter Sayuri, and the two of them have twin daughters named Kaede and Erika. The result is a rather complex family tree that spans five generations.
The New Retcons: The fic ends with this after multiple revelations and twists:
Elly had four children by three different fathers: Claire with Frank Day (who was Happily Adopted), Michael with Stan Watson (who was the result of her putting The Baby Trap on Stan, who divorced her shortly after the birth) and Elizabeth and April with John Patterson. In addition, Claire shares a birthday with April and is old enough to pass off as her mother due to Claire being a Teen Pregnancy and April being a later in life pregnancy.
John later fathers twin sons Vernon and Ellis with Kortney, who’s a few years older than April, making the boys old enough to be the sons of either of their sisters.
Elizabeth has a branch of her own: her son James is actually the result of a one night stand with her ex boyfriend Warren Blackwood during her honeymoon with Anthony.
A Thing of Vikings: Being a small tribe of only a few hundred people, most of the Hooligans are related by blood, adoption, or marriage to each other, and the (incomplete) family tree on the wiki is extremely tangled. See also the epigraph for Chapter 122, which mentions the complications involving same-sex and polyamorous relationships.
probably takes the cake for shocking Luke, I Am Your Father plot twists—Snape is revealed to be Harry's father but it gets even more complex with the revelations that Snape is actually a Potter by birth and was James' twin brother but he was blood-adopted out to the Potter's family friend Eileen Prince and her husband. Also Lily isn't a muggleborn but she was adopted out to her squib relatives because her parents feared she would be targeted seeing as she is the daughter of McGonagall and Voldemort.
