One possible plot could be that Naruto and his sister are sent on a mission to a foreign land where English is the main language. They struggle at first to communicate but then start learning English from the locals. Along the way, they use their new language skills to befriend the people and gain important information for their mission.
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.
Their relationship can be shown as she teaches Naruto English. For example, if Naruto gets a word wrong, she might gently correct him with a teasing smile, which shows their sibling - like closeness.
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.
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.
One idea could be that Naruto's OC sister is a prodigy in English. She uses her language skills to communicate with foreign ninjas and gain valuable information for the village. This could lead to Naruto being proud of her and also learning some English from her in a fun, brother - sister interaction way.
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.