There could be. Fanfic writers love to experiment with different character connections. Maybe in this fanfic, the cousin relationship between Harry and Caroline brings out new adventures or changes in the magical world we know from the original Harry Potter series. However, it might not be a very common concept, but with the large number of fanfics out there, it's definitely a possibility.
Yes, there are likely to be such fanfics in the vast world of fanfiction. You can try searching on popular fanfic websites like Archive of Our Own or FanFiction.net. Just use the relevant keywords in the search bar, and you might find some interesting stories exploring the relationship of Harry being James' cousin.
One possible plotline could be that Harry discovers his connection to Buffy and has to navigate the world of slaying while also dealing with his own magical world in Harry Potter. Maybe he uses his magic to help Buffy in her battles against vampires and demons in a way that no one else can.
In a more detailed scenario for this fanfic, the 'turn' could refer to a plot twist. For example, Buffy is in a normal day in her world when suddenly there's a magical turn of events that links her to Harry's magical realm. Regarding her hair color, it could be that the fanfic writer uses it as a symbol. Say, when she starts to understand and use some of Harry's magic, her hair color changes to a golden hue, representing her newfound power or connection to the magical side. This fanfic has the potential to blend the two different universes in a really creative way.
In one concept for this fanfic, the 'turn' is a time - related event. Harry accidentally sends Buffy back in time, and during this journey, her hair color changes depending on the era she's in. In the medieval times, it could be a dark black to blend in. In a more futuristic time, it could be a neon color. This fanfic could explore how Buffy adapts to different time periods while also dealing with the changes in her hair color.
A 'Harry Potter is Caroline's cousin' fanfic could have a lot of potential. It could start with the discovery of their relation. Maybe Caroline has been living in a different part of the wizarding world, and when they meet, they have to adjust to having a new family member. Their personalities might be different, but their family bond could bring them closer. The fanfic could also introduce new magical elements or creatures related to Caroline's family side. It might also explore how the wizarding society views their family connection, whether it's seen as an advantage or a liability for both of them. There could be adventures and challenges that they face together, different from what Harry experiences with Ron and Hermione.
For Carrie, you can focus on her internal struggle with her powers. Show her moments of fear, confusion, and also the times when she starts to feel a sense of control. For Harry, you could make him very curious and protective. He might be the one who is always asking questions and trying to keep Carrie safe.
There are many fanfic communities out there. Some popular ones might have the 'harry is buffy's brother fanfic' you're looking for. You could also ask in relevant forums or groups dedicated to either the Harry Potter or Buffy the Vampire Slayer fandoms. Fellow fans might be able to point you in the right direction. For example, Reddit has some great sub - forums for these types of things.
A plot might involve Harry being raised by different relatives because of this cousin relationship. Maybe he grows up in a different environment and has different values. When he enters Hogwarts, his interactions with the other characters are very different. For example, he might be more confident or more reserved depending on how his new family treated him. And this would create a whole new set of relationships and conflicts within the story.
Well, in a 'harry raises his cousin fanfic', it could involve Harry having to take care of his cousin in a situation where the normal family structure has broken down. Maybe his aunt and uncle are unable to look after the cousin for some reason, like they've been put in a magical coma or something. Harry then has to step up and teach his cousin about the wizarding world, how to handle magic, and also about normal life things like school and making friends. He might face difficulties because his cousin could be resistant to the new world he's being introduced to, or Harry might struggle with his own insecurities about being a good caregiver.