In a work, there are two characters who aren't related in anyway. 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 for a twist, or because the characters have a lot in common to justify that decision.
Once again sorry about the bad summary, but I think you get the point.
If there's any better titles, feel free to type it.
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.
- 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.
- Spectacular Spider-Man 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.