One way it could be developed is that his sister might be jealous of Naruto's growing popularity or skills. Maybe she feels overshadowed and starts to bully him by spreading false rumors about him in the village. This could lead to Naruto feeling more isolated and misunderstood, even as he tries to prove himself.
The sister could be bullying Naruto out of envy. If Naruto has some special abilities or talents that she doesn't, she might be jealous and lash out at him.
Yes, there might be some. The Harry Potter fandom is quite large and diverse, so there could be fanfictions exploring such darker or complex themes within the Potter family. However, this goes against the general perception of James Potter as a heroic figure in the original series.
In the Naruto series, Naruto doesn't have a biological sister in the original story. So in this fanfiction, the author probably creates a new character to be his sister. It could be an original character with unique abilities and a complex backstory that leads to the situation where Naruto dies by her hand.
Well, perhaps in the fanfiction, James Potter's sister has a special gift or ability that James is envious of. This envy turns into bullying as he tries to undermine her confidence in using that gift. For example, if she's really good at Potions and James isn't, he might try to sabotage her Potions experiments. Eventually, his sister could find some allies who help her stand up to James, and through a series of events, James has to face the consequences of his actions and learn to accept his sister for who she is.
She could be an original character created specifically for this fanfiction. The author might give her unique abilities and a distinct personality to make the story more interesting.
One possible reason could be to protect her. Naruto might believe that by leaving, he can draw danger away from his sister. For example, if there are powerful enemies targeting him, he doesn't want his sister to get caught in the crossfire.
The relationship could start with Naruto being overly protective of his sister, but she wants more independence. This causes friction. Later, when she gets into trouble that only Naruto can help her with, he steps in. This makes her realize that his over - protectiveness comes from love, and they begin to repair their relationship. For instance, if she is kidnapped by a powerful enemy, Naruto uses all his skills to rescue her, and during this process, they have heart - to - heart conversations.
One idea could be that their own village betrays them. Maybe there's a power - hungry leader who sees Naruto and his sister as threats to their new regime. They could be framed for a crime they didn't commit and then forced to flee the village. In the process, they might have to rely on some unlikely allies they meet on their journey, like a group of outcasts who also have a grudge against the village.
She could be very sad. After all, Naruto is likely an important person in her life. She might feel abandoned and start to question why Naruto left her.