One possible motive could be jealousy. Maybe the sister is jealous of Naruto's popularity, his strength, or the attention he gets from others in the village.
I'm not sure if there is such a specific fanfiction. Fanfiction varies greatly in content and themes, and while it's possible for someone to create such a dark and out - of - the - ordinary plot in the Naruto fanfiction universe, I haven't come across it personally.
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.
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.
It could bring them closer. Having to learn and speak English together would mean they rely on each other more. They would share the struggles and the joys of mastering a new language, which would strengthen their bond.
It could be that the family discovers a false prophecy about Naruto bringing doom. In the fanfiction, they might be so desperate to prevent this 'doom' that they decide to kill Naruto, not realizing it was a false prophecy all along. This would add a tragic element as their fear led them to commit an unthinkable act.
The impact would be huge. For one, all of Naruto's friends like Sasuke and Sakura would be devastated. Their whole world view which was centered around Naruto's positive influence would be shattered. Also, the village of Konoha would lose its greatest hope for the future. Naruto was seen as the one to bring about change and peace, so without him, the village might descend into chaos or be more vulnerable to outside threats.
This is not an acceptable theme at all in Naruto fanfiction. Incestuous relationships go against basic moral and ethical norms, and Naruto's world also adheres to certain values. Such a theme would be considered very inappropriate and deviate from the positive and family - like relationships depicted in the original Naruto series.
Well, some of his friends could be really curious about his sister. They might start including her in their group activities. Naruto would then have to balance his time between his friends and his new - found family relationship with his sister. His friends might also learn a lot from his sister if she has some special skills or knowledge.