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.
In the fanfiction, it could be that his sister is bullied herself by others in the village, and she takes out her frustration on Naruto. She could be mean to him, steal his things or exclude him from family activities. Naruto, being the kind - hearted person he is, might initially be confused and hurt. But as the story progresses, he could try to help her deal with her own problems, while also standing up to her bullying in a non - violent way.
Perhaps his sister bullies him because she has been influenced by the negative views some in the village have of Naruto. She could constantly put him down, mock his dreams of becoming Hokage. Naruto, in this situation, would have to find the strength within himself to rise above her bullying and still pursue his goals. He might try to reason with her, showing her that he's not the person others think he is.
One way could be that his parents were forced into it. Maybe they were under some powerful dark magic or mind - control that made them seem to betray Naruto. For example, an evil villain could have manipulated them to make Naruto think they don't care about him.
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.
Well, in some extreme fanfictions, it might be due to some kind of mind - control or dark magic that forces Naruto to do such a heinous act. But this goes completely against his character in the original series.
In this fanfiction scenario, it could be that an enemy uses some sort of mind - control jutsu on Naruto's family. The family, under the control of this jutsu, doesn't recognize Naruto as their own and kills him. This would add an element of tragedy as Naruto would be killed by those he loves and trusts the most without them being fully in control of their actions. It would also open up the story for a revenge plot by Naruto's friends or other allies against the enemy who controlled his family.
Perhaps his parents were misled to believe that Naruto was a threat to the village. They might have been fed false information about Naruto being a potential danger, like being a vessel for a great evil. So, in an attempt to protect the village they loved, they made the tragic decision to kill him.
They could start by having some friendly competitions. For example, in a race to the top of a mountain or a sparring match. This would not only show their bond but also their competitive nature as siblings.
One way could be that Naruto discovers it's all a big misunderstanding. Maybe someone is manipulating the situation for their own gain, and once he uncovers the truth, he can put a stop to the forced marriage.
In this fanfiction, perhaps Naruto's sister has some special power. Naruto loves her not only because she's family but also because he sees the good in her power. He might teach her how to control it and use it for good. They could go on adventures together, with Naruto always being the over - protective brother and making sure she doesn't get into too much trouble.
Perhaps in this fanfiction, his sister has done something wrong or dangerous, and Naruto has to 'beat' her in the sense of stopping her. It could involve a moral or ethical conflict within the story that Naruto has to resolve by overpowering his sister's actions.
Well, it could be about Naruto and his sister having different loyalties. Say, Naruto is loyal to the village, but his sister has a connection to an outside group that Naruto doesn't trust at first. They end up in a conflict until they realize they can work together to achieve a greater good for both the village and the outside group's worthy cause.