No. Incestuous relationships are generally not considered acceptable in most fanfiction communities. The Naruto series promotes positive values and family bonds that are normal and healthy. Having such a plot goes against the general moral and ethical standards that the Naruto fanfiction community typically adheres to.
Naruto would be a very loving and supportive father. He'd probably be a bit overprotective at times because of all the dangerous things he's faced. But he'd also encourage his child to be independent and follow their dreams. He'd be there to teach them ninja techniques and share his own experiences to help them grow.
If in fanfiction Naruto had a child with his sister, it would be a radical departure from the original Naruto storyline. In the original, Naruto's focus is on becoming Hokage, protecting the village, and his relationships with his comrades. This new scenario would introduce a moral and ethical dilemma that the original story never had. His relationship with his sensei, like Jiraiya, and his peers would become very strained. His family, which has always been presented in a positive light, would now be associated with something very wrong. It would also change how the fans perceive Naruto, as he is now involved in an unacceptable relationship that goes against the very essence of the character and the world he lives in.
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.
One common storyline could be Naruto and his sister being separated at a young age, and then Naruto spends a lot of time searching for her. When they finally reunite, his love for her is even stronger, and he vows to never let her go again.
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.