In a work, there are two characters who aren't related in any way. The creator may not have intended (at the time, anyway) for them to be relatives. But when that work is adapted to another form of media, suddenly the people in charge decided to make them relatives. This can be for a variety of reasons including (but not limited to):
- A pragmatic change to eliminate some exposition; it's easier to introduce Alice and Bob as brother and sister than to introduce Bob as Alice's childhood friend staying at her house while his parents are off being the Hero of Another Story.
- The characters have something that makes them very similar (be it Superpowers, appearance, motivations, what have you), and thus linking them together makes sense thematically.
- By making the characters relatives the stakes are raised and there's a more personal connection between them than if they were just friends.
- Avoiding the Unfortunate Implications of an adult being One of the Kids by making said adult a parent to at least one of the kids they're running around with.
- Trying to stomp out homosexual undertones in the original work by making the affection characters share familial instead.
- Adding a twist to a remake of a story that is subject to It Was His Sled.
- The result of creating a Composite Character.
- Ultimate Spider-Man:
- in the main Spider-Man comics the name Spider-Woman has been used by a number of ladies, all of whom have almost nothing do with Spider-Man other than similar powers. In Ultimate Spider-Man Spider-Woman is Peter Parker's Opposite-Sex Clone.
- Inverted with Ben Riley who is Peter's clone in the main comics (sometimes referred to as his "brother" or identical cousin), but is an unrelated lab assistant in the Ultimate verse.
- In the Ultimate verse Liz Allan is made a Composite Character and thus is not only the alter ego of Fire Star, but The Blob is her now her uncle.
- Spider-Man: Chapter One makes Norman Osborn and Sandman cousins to explain their similar looking hair. Became this trope once the book was (very quickly) declared out of continuity and simply an alternate reality take on the Spidey mythos.
Live Action TV
- In Mortal Kombat: Annihilation, Raiden and Shao Kahn are revealed to be brothers. And later it's revealed that Shinnok is their father. Which isn't canon in the video games, except for the equally non-canon Mortal Kombat vs. DC Universe.
- May have been intended in the Transformers film franchise, when Optimus Prime calls his arch-nemesis Megatron "brother", even thought they're robots. Making this example a sort of Cain and Abel.
- In Batman & Robin, Batgirl is made Alfred's niece (instead of being Commissioner Gordon's daughter).
- Kirsty in Hellraiser was originally a neighborhood love interest for Larry/Rory Cotton in the novella The Hellbound Heart. In the movie she becomes his daughter.
- The Doctor Who film Dr. Who and the Daleks had Susan and Barbara as Dr. Who's granddaughters (with the surname "Who" no less), with Ian being Barbara's boyfriend.
- In the TV series of The Addams Family, Fester is Morticia's uncle. In the films, he's Gomez's brother.
- X-Men Origins: Wolverine makes Sabertooth Wolverine's half-brother (as opposed to simply being another Weapon X experiment). Though it seemed to be fanon before the movie was filmed, due to Dog (Wolverine's half-brother in the comic) having a resemblance to Sabertooth in the end. This makes him a Composite Character.
- In Arrow, Shado is the daughter of Yao Fei. The comic book counterparts have never even met. Also Thea Dearden Queen is basically Mia Dearden reinvented as Ollie's sister.
- In the Sky1 adaptation of Going Postal, "Princess", the very young clacks tower operator, becomes the granddaughter of Grand Trunk engineer Mr Pony.
- The David Suchet adaptation of the Hercule Poirot novel Cards on the Table by Agatha Christie adds a mother-daughter relationship between two of the suspects, partly in order to make the older woman's protection of the younger woman more plausible.
- Freddy Martinez and Cara, best friends in the Goosebumps book "Vampire Breath", are said to look like siblings. When it was adapted into a TV episode, they are actually siblings. They learned that they're vampires and Count Nightwing is their grandpa. While in the book, only Fred is a vampire, while Cara becomes a werewolf...
- In the stage musical of Disney's The Little Mermaid, Triton and Ursula are siblings. (This was part of Disney's original concept for the movie, but it was dropped in development.)
- Cheshire in Young Justice becomes the sister of Artemis, and therefore the daughter of the original Huntress and Sportsmaster. This entails Artemis and Huntress getting a Race Lift.
- In Martin Mystery the main character and his female partner in paranormal investigating are step siblings. In the comics the show is based off of she's his unrelated Love Interest.
- The Spectacular Spider-Man:
- In the Spider-Man comics Silver Sable and Silvermane are a completely unrelated mercenary and gangster. In The Spectacular Spider-Man Silver Sable is Silvermane's villainous daughter. Thus her real name was changed from Silver Sablivona to Sable Manfredi.
- The series also combines this trope with Composite Character. In the comics Uncle Ben's killer is a nameless street thug. In the cartoon he's Black Cat's father The Cat.
- Molten Man gets upgraded from Liz Allan's step brother, Mark Raxon, to a full brother. Accordingly, he gets a last name change to Allan and a Race Lift to match the one Liz got for the series.
- Spider-Man (1967) had Mary Jane Watson as Captain Stacy's niece. Which of course would have also made Gwen Stacy her cousin. Unfortunately, we never get to see Gwen, so we don't get to see how being related would have reflected on their usual Betty and Veronica dynamic.