One key theme could be family dynamics. Since Naruto is neglected, it might explore how his sister could potentially change his situation or their relationship. Another theme could be self - discovery. Naruto may find new aspects of himself with his sister's influence. Also, it might touch on the theme of redemption, perhaps for those who neglected Naruto in the first place.
In the canon story, Naruto doesn't have a sister. So, right away, this is a big difference. In the fanfic, the sister character can change the course of Naruto's life in ways that don't happen in the original. For example, she might be able to prevent some of the bullying he endures.
In this type of fanfic, it could be that Naruto's sister is ill or has some special condition that requires a lot of care from their parents. So, Naruto is pushed to the side. However, Naruto being Naruto, he still tries to find his own way in the ninja world. He might find a mentor outside of his family who sees his potential and helps him grow. Eventually, he could come to terms with his family's situation, or they could realize that they've been neglecting him and try to make amends.
It's popular because it explores a different side of Naruto's story. The idea of him having a sister who cares when others don't adds an interesting dynamic. People like the concept of family loyalty and the sister being a support system that Naruto was lacking in the original story.
Naruto would likely become more independent. Since he's being neglected, he has to rely on himself. He might become a bit more of a lone wolf at first, not trusting others easily as he feels his own family has let him down. But this could also make him stronger in the long run. He'd have to figure out things on his own, like how to control the Nine - Tails' power without much family support.
When writing a neglected Harry x sister fanfic, consider the setting carefully. The wizarding world has its own rules and dynamics which can play into the neglect and the relationship. You could have the sister be a Squib, for instance, which might lead to her being neglected. Then, plot out a series of events that will force Harry and his sister to interact. Maybe they have to team up to solve a mystery. As you write, use dialogue effectively to show the growth of their relationship. For example, in the beginning, their conversations might be short and full of tension, but as they get closer, the dialogue becomes more open and affectionate.
Well, in some fanfictions of this kind, Naruto's sister is often portrayed as having a special bond with him. She might have a unique ability that she uses to shield Naruto from the emotional pain of being neglected. For example, she could have a power to absorb negative emotions, and whenever Naruto is feeling down because of the neglect, she uses this power to make him feel better. There are also stories where she tries to expose the unfair treatment Naruto is getting to the village elders, but has to be very careful as she doesn't want to put Naruto in more danger.
One possibility could be an original character created specifically for the fanfiction. This gives the writer the freedom to shape her personality and backstory completely. Another could be a minor character from the Naruto series that doesn't get much attention, like Tenten's sister if she had one. It could also be a reimagined version of Hinata as Naruto's sister. Since Hinata has her own story in the original series, reimagining her as Naruto's sister with a neglected past could lead to some interesting fanfiction ideas.
There's also 'Sibling's Path in Konoha'. This fanfic delves deep into how Naruto and his sister interact within the village of Konoha. It shows their daily lives, their training together, and how they support each other against the various threats that come their way. It's a great read if you like stories that focus on family bonds within the Naruto universe.
She might feel guilty. Since they are siblings, she could be aware of Naruto's neglect and try to make it up to him in small ways, like sharing her food or secretly leaving him little gifts.