The sister in this fanfiction is likely a fictional creation for the purpose of the story. She could be a character with a dark past or a special mission. It might be that she was brainwashed by some evil force to kill Naruto. So she's not a character from the original Naruto lore but a new addition to this particular fanfiction world.
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.
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.
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.
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.
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.
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.
The story could progress with his sister seeking revenge if Naruto was killed maliciously.
Maybe she's jealous of Naruto's popularity or his strength. It's possible that she feels overshadowed by him all the time and that jealousy turns into resentment, leading her to hurt Naruto.
Naruto's love for his sister would probably add a new layer of complexity to his character. On one hand, he would be the strong and fearless ninja. But on the other hand, he would have a soft spot for his sister. This could lead to some internal conflicts. For example, if he has to choose between a mission for the village and protecting his sister. It would also force him to grow in terms of his decision - making skills and his ability to balance different aspects of his life.