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.
In such a fanfiction, Naruto could die in various ways. Maybe his sister is somehow possessed and attacks him fatally. Or there could be a situation where she accidentally causes his death during a fight due to a lack of control over her power.
In this kind of fanfiction, his sister would probably be devastated. She might go through a long period of grief and could potentially vow to avenge Naruto's death if it was caused by someone.
In this fanfiction, it could be that Naruto's sister is being targeted because she has information that could expose a powerful and corrupt group. Naruto, being the selfless hero he is, shields her from their attacks. His death becomes a symbol of resistance against this group, and his sister then takes on the task of bringing them to justice.
One possible storyline could be that Naruto's sister is being targeted by a powerful and vengeful enemy. Naruto, being the selfless hero he is, throws himself in front of a deadly attack meant for her. His death could then inspire his sister to take up his ideals and become a great ninja in her own right. Another idea is that there is a dangerous mission to protect a secret related to their family that only his sister can safeguard in the future. Naruto sacrifices himself to ensure she can complete this important task. And perhaps in his last moments, he passes on his most powerful jutsu or some important knowledge to his sister to help her in her future battles.
His death would be a huge shock to the ninja world. Naruto is a well - known and respected figure. His sacrifice for his sister would be seen as an ultimate act of heroism. It could inspire a new generation of ninjas to be more selfless and protective of their loved ones. Also, the villages that Naruto had strong relationships with, like Konoha, would likely hold memorials for him and his story would be passed down as a cautionary and inspiring tale.
The Hidden Leaf Village would be in deep mourning. His friends like Sasuke would be filled with a mix of anger and grief. They might vow to protect Naruto's sister in his memory. The village would also probably hold a grand memorial for Naruto, and his story would become a legend to inspire future generations of ninjas.
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.