It could be an original character created just for the fanfiction. Many fanfiction writers like to introduce new characters to add their own unique twists to the story.
If such a fanfiction exists, one possible reason could be a mind - control situation. Maybe an enemy uses some powerful genjutsu to make Naruto believe his sister is a threat, causing him to kill her accidentally. Another reason could be a misinformation plot where Naruto is tricked into thinking his sister has committed some unforgivable crime against the village.
I'm not aware of any official 'Naruto kills his sister' fanfiction. This goes against the general nature of Naruto's character who values family and friends. Fanfiction that involves such extreme and out - of - character scenarios might be very rare or non - existent in the mainstream Naruto fanfic community.
He would likely be seen as a traitor by the village. The villagers, who once saw Naruto as a hero - in - the - making, would now turn against him. He'd be hunted by the ninja forces of Konoha.
In the end, Third Sister got together with the warm man, Wu Jianfu, and gave birth to a girl. But in the end, Wu Jianfu sacrificed himself. His third sister became a widow and raised the child alone.
In Dream of the Red Chamber, the third sister was Jia Xichun. If you're interested in Jia Xichun, I can recommend the historical novel " Dream of the Red Chamber: I'm a Hereditary Champion ". It tells the story of the main character, Jia Heng, who transmigrated into the Dream of the Red Chamber and became Jia Jing's son, Jia Heng. He obtained the path of cultivation, began to change his fate, and made a name for himself in the Dream of the Red Chamber. I hope you like my recommendation.
Most of the fandom would probably be horrified. Naruto is known for his positive values, and killing a family member is completely against that. There would likely be a lot of criticism towards the author of such a fanfic.
At first, Sasuke might not believe it was Naruto's own will. He would try to find out if Naruto was under some sort of external influence. But if he found out that Naruto was fully responsible, Sasuke's reaction would be one of pure hatred. He would abandon all his previous beliefs and goals and make it his sole mission to destroy Naruto. He might also cut off all ties with the village that Naruto was protecting, seeing it as an accomplice in his sister's death.
One consequence could be that the wizarding world is in shock. Dementors are usually just sent away or repelled, not killed. So, the Ministry of Magic might be very concerned about this new development and start to investigate Harry.