It could also be society in the context of the fanfiction. If the sister is trying to break free from traditional norms and change for the better in a way that goes against what society expects, then society as a whole could be the antagonist. For example, if she wants to pursue a career that is not considered 'feminine' in that fictional society, she might face a lot of opposition.
It could be another sister. Sometimes in stories, sibling rivalry can be a strong source of conflict. The other sister might be jealous of the main sister's growth and change for the better and try to undermine her at every step.
Morgana could be a great antagonist. She might see Merlin's sister as a threat to her own plans for Camelot. Maybe she discovers that Merlin's sister has a power that could undermine her magic or her influence over the court.
It could be an old enemy of Percy's father, Poseidon. Maybe this antagonist blames Percy and his sister for something Poseidon did in the past and wants revenge on the family. So they target Percy's sister as a way to get to Poseidon through his children.
It could be a rogue Death Eater who survived Voldemort's fall and blames Harry and his sister for the downfall of their cause. This Death Eater might be more cunning and powerful than the others, and he targets Harry's sister specifically because she represents a new threat to their dark plans. He might try to use her as a pawn to draw Harry out and then destroy them both.
A powerful and greedy Pokémon collector could be the main antagonist. He might try to steal Ash's Pikachu or other strong Pokémon from them. He would constantly be one step ahead, setting up traps and obstacles for Ash and his sister during their travels.
It could be an ancient monster that has a grudge against the family of Poseidon. Since Percy and his twin are children of Poseidon, this monster might target them specifically. Maybe it's a sea serpent that was once defeated by Poseidon and now wants revenge by going after his descendants.
It could be a new, super - intelligent animatronic created by combining the technology from both Sister Location and the original FNAF series. This animatronic would have all the worst traits of both, like the stealth of the Sister Location ones and the aggression of the original ones.
Maybe a time - traveling villain from a different dimension who sees Lewis' sister as a threat to his plans of ruling all timelines. He constantly tries to thwart her efforts to make the future a better place. For instance, he might try to disrupt her attempts to introduce new technologies to the Robinson family that could benefit everyone. He would set up various traps and obstacles for her throughout her adventures in different time periods.
A mischievous fairy who lives in their backyard could be an antagonist. She might be jealous of the attention the kids get and start causing trouble for the little sister. Like making her toys disappear or creating illusions to scare her. This would force the little sister to be brave and find ways to outwit the fairy with the help of her brothers (even if they don't initially believe her about the fairy).
It could be the sister herself in a sense. Her over - the - top fan behavior might cause so many problems that she becomes like an antagonist to the normal, smooth - running of the Stark family and the superhero world around them.
It could be a member of Naruto's family who is fueling the neglect. Maybe an older relative who believes that the sister has more potential or is more worthy of attention and resources. This person could actively try to keep Naruto down.