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- It makes for a more pragmatic and realistic story. If Bob constantly stays over at Alice's house in the original, it's easier to just make them siblings in the new take on things.
- The characters are very similar, be it due to their appearance and/or personality, and it just makes sense.
- Making the older character related to a younger one takes out a good amount of the Unfortunate Implications.
- Tie the characters together emotionally.
- One of the characters is a Composite Character and this is a side-effect.
The opposite of this is, unsurprisingly, Unrelated in the Adaptation.
- In DC Comics' New 52, Wonder Woman is the demigod daughter of Zeus and the aunt of Wonder Girl.
- Might be a surprise considering what a hallmark of the comics it is, but Sif and Heimdall weren't related in the original myths, while the comics have almost iconically cast them as brother and sister.
- In The Ultimates, Thanos is Ronan the Accuser's father.
- In Transformers: Robots in Disguise, Galvatron and Arcee are twins. They don't look alike because, while Arcee was born male and near-identical to Galvs, she had a sex change and adopted her pink colours.
If we're honest, this trope is most prevalent in fanfictions. In Crossovers, this is usually done to quickly allow two different groups to quickly interact. In these cases, the characters are usually cousins. If the characters were played by the same actor, they're sometimes long-lost twins or just cousins who are able to pass off as twins.
- A common one is making Buffy Summers and Scott Summers cousins, based off of them sharing their last name.
- In every DC/Marvel crossover ever, expect Namor and Aquaman to be cousins.
- In The Elements of Harmony and the Savior of Worlds, FiM Spike is revealed to be the grandson of G1 Spike.
- In Disney School, Ariel and Hercules are cousins based off of mythological connections (Ariel's father is Triton, the mythological son of Poseidon, who is Hercules' uncle).
- In Child of the Storm, Everyone Is Related. And we do mean everyone. Beyond telling you that Harry Potter is Thor's son, we'd be giving out massive spoilers.
- In Changing of the Guard, Kevin Levin was legally adopted by Max Tennyson.
- Defenders of the Universe features this in both its crossover and non-crossover aspects:
- White Diamond created Yellow, Blue and Pink from the same type of comets that Alfor used to create the Voltron lions (in fact, in Pink's case, it was the leftover material of the same comet), making the Diamonds connected to Voltron.
- In this AU, Keith is a full-blooded Galra whose father is Kolivan, while Lance is an Altean and Allura's cousin.
- In many a Zoey 101/Victorious crossover, Lola Martinez and Tori Vega are cousins. When Shelby Marx shows up, she's usually Tori's (sometimes long-lost, sometimes always having been there) twin sister, if not a Composite Character with Tori. It's weird in that, although Victoria Justice is not the only actress to appear in more than one Nick Verse sitcom, she's the only one whose characters the authors feel the need to connect.
- As said below, after the 2007 Transformers movie, some writers refuse to even countenance the notion that Optimus Prime and Megatron aren't long-lost biological brothers.
- Some Transformers Prime writers like to think that Optimus Prime and Ultra Magnus are brothers. Not a hard leap, considering how similar they look.
- Starscream, Thundercracker, and Skywarp are usually written as brothers. When it comes to the other Seekers, Slipstream is the most common one to pop up as their sister, but it's not unheard of for Starscream to come from a large family.
- Related to Alternative Character Interpretation, some fans took Keith's declaration of Shiro being his brother in Season 6 of Voltron: Legendary Defender literally and write the two as half-brothers.
- Shows up a lot in works based off of My Little Pony: Friendship Is Magic. The most common are making Twilight a surrogate mother to Spike, Rainbow and Scootaloo being sisters (adopted or biological), Sunset Shimmer having been adopted by Celestia, and the Dazzlings being sisters.
- Hermione can sometimes be revealed as a descendent of Rowena Ravenclaw (completely missing the point of Hermione's character) or a distant relation to the Malfoys. The latter usually pops up in Draco/Hermione fics.
- In many Time Lady!Rose fanfics, Rose, or Arkytior ("Rose" in Gallifreyan), is the Master's sister.
- In some Time Lady!Clara fanfics, Time Lady!Sarah Jane can show up as her mother.
- In Human!AU Steven Universe fics, Amethyst and Jasper will often be biological sisters.
- In the Danganronpa 2: Goodbye Despair fanfic New Island Life, Kazuichi Souda and Hiyoko Saionji are cousins, with Hiyoko's father and Kazuichi's mother being brother and sister.
- In the Sailor Moon fic series Sailor Scout Sacrifice, Minako and Motoki are brother and sister.
- Several fics back in the day used to make Rei and Mamoru siblings or even twins in the Silver Millenium. One fic even had them discover this as their present-day selves, resulting in a horrified reaction when they remembered that they'd almost had sex once. (They got over it quickly enough, though, and embraced the idea of being family.)
- In Dr. Who and the Daleks, Barbara is both Dr. Who's granddaughter and Susan's older sister.
- Princess Anna and Queen Elsa in Frozen. In the original work, The Snow Queen, Gerda and the Snow Queen were unrelated: Gerda was a human girl and the best friend of a boy (Kai) enchanted and kidnapped by said Queen, who was closer to The Fair Folk than to a human. While Anna and Elsa were originally unrelated, Disney ultimately wound up going for the sisters route.
- For what it's worth, the fandom readily embraced the idea of the Queen and the Muggle Best Friend being lovers.
- One of the most criticized, and notable, aspects of Hercules is that Hera is the hero's biological mother whereas his actual bio mom in the myth, Alcmene, was Herc's foster mother in the movie.
- Ariel and Ursula (maybe) in The Little Mermaid. In the original text, the Sea Witch was totally neutral and had no ties to anyone under the sea. In some early drafts of the film, Ursula was Ariel's evil aunt and this sometimes pops up in ancillary media.
- Marvel Cinematic Universe:
- In order to avoid paying the Sax Rohmer estate (and in order to avoid using a character riddled with Unfortunate Implications), the Mandarin will take Fu Manchu's place in Shang-Chi and the Legend of the Ten Rings as Shang-Chi's Archnemesis Dad.
- Erik Killmonger is Black Panther's paternal cousin.
- Egghead is Ghost's father in Ant-Man and The Wasp.
- Tony and Howard Stark is an interesting case. Tony is Howard's biological son but after the films came out, Tony was revealed to be Howard's adopted son. Absolutely no piece of media has ever touched this.
- Peter Quill is still a Half-Human Hybrid, but this time around, his father is Ego the Living Planet, not J'Son of the Spartax.
- Hela is a twist on this. Here, she's presented as Odin's biological daughter, making her related to both him and Thor, but making her unrelated to Loki, who is her biological father is the comics and the original myths. 
- In the comics, Nebula claimed to be Thanos' granddaughter, a statement that was never proven but was likewise never disproven. Here, she's firmly established as his "adopted" daughter. The same for the Black Order. They were always associated with Thanos in the comics, but this is where they became "family".
- Likewise, Corvus Glaive and Cull Obsidian (the renamed Black Dwarf) are confirmed by Word of God to be biological siblings.
- In the book, Wilbur and Lewis are best friends. In Meet the Robinsons, Wilbur is Lewis' Kid From the Future.
- Rudolph the Red-Nosed Reindeer had no stated parents in the original text. In the iconic Rankin/Bass Productions special, he's Donner's son.
- Transformers might have been leaning towards this. At the end of the film, Optimus refers to Megatron as his brother. Later media made it very clear that Optimus was speaking figuratively, but some early ancillary media related to the film cast them as biological brothers.
- In X-Men: First Class, Mystique is retconned into being Charles Xavier's adoptive sister. Not only does this relationship not exist in the comics, it didn't even exist in the movies before First Class.
- In Jessica Jones, the hero is the adopted sister of Patsy Walker, a character that she barely ever interacted with in the comics.
- In Merlin, Morgana is still Arthur's half-sister but their shared parent is Uther, not Igraine. By consequence, Unrelated in the Adaptation applies to Morgana and Igraine.
- In Riverdale, the Blossoms and the Coopers are related, specifically Betty and Cheryl are third cousins, owing to both girls' great-grandfathers having been brothers.
- In Supergirl, General Non is the eponymous heroine's uncle. Albeit not a blood relative.
- Once Upon a Time has many fairy tales (with altered plots to at least some extent), which includes making characters from originally unconnected stories related.
- Peter Pan is the father of Rumplestiltskin, who is a Composite Character of at least four different characters from different stories - the other important one is the Beast Prince from Beauty and the Beast.
- The Wicked Witch of the West from Wizard of Oz and the Evil Queen from Snow White are half-sisters. Their mother is the Queen of Hearts.
- The Snow Queen and Gerda from The Snow Queen are sisters. The latter is also the mother of Elsa and Anna from Frozen.
- Ursula is the daughter of Poseidon.
- The second version of Rapunzel is also the second version of Lady Tremaine, making her related to Anastasia and Drizella Tremaine.
- The second version of Cinderella's father is the biological father of Anastasia and Drizella instead of being their stepfather. However, he is in fact not related to Cinderella, instead being her stepfather and the only father she knew.
- One of the two versions of Alice is the daughter of Mother Gothel and an alternate Captain Hook-
- In the original Mortal Kombat timeline, Raiden the Tunder God and Fujin the Wind God were simply fellow deities and allies. In the new one, they actually are brothers on top of being Gods.
- Rarity and Sweetie Belle are sisters in My Little Pony: Friendship Is Magic.
- In the Silver Surfer animated series, Thanos is the Mentor's brother rather than his son. Hand to god, this is due to a typo.
- In The Spectacular Spider-Man, the random mugger that killed Uncle Ben is Black Cat's father which makes him a Composite Character with The Cat.
- Believe it or not, the Thirteen Primes in Transformers. Prima, Vector Prime, Alpha Trion, Nexus Prime, the Fallen, and the Liege Maximo were all quasi-divine Cybertronians but unconnected to one another. When the idea of the Thirteen began to come together, these characters were assigned to the group.
- The same actually goes for Primus and Unicron. In the original comic, Unicron came first and Primus was an unconnected entity born to oppose him. Every other adaptation casts them as twin brothers.
- As a result of Composite Character, Haggar in Voltron: Legendary Defender is Lotor's mother.
- In the original The Legend of Spyro continuity and the Skylanders main series, Malefor ordered his henchmen to steal Cynder's egg and corrupt her to be evil. In Skylanders Academy, Cynder is Malefor's daughter.
- The title character of Harvey Birdman, Attorney at Law is an ancestor of George Jetson.