He would be filled with rage. Naruto has a strong sense of justice and the fact that someone took Hinata away from him would fuel his anger. He would vow to find and punish whoever was responsible. He might even become reckless in his pursuit of vengeance, endangering himself and those around him. His friends would have to step in to try and bring him back to his senses.
There could be a scenario where Naruto sacrifices himself to save Boruto or the entire Hidden Leaf Village from a catastrophic event like a massive attack by an unknown and dangerous force. He might use all his chakra to protect the village, leaving him with no energy to survive.
Yes, there might be such fanfics in the vast Naruto fanfic community. Some fanfic writers like to create alternative scenarios where the characters' identities are swapped or unexpected events like the main character's death occur. These fanfics often explore different aspects of the Naruto universe and the characters within it.
He would be filled with a deep sense of grief. Naruto has always cared about Hinata, and her death would be like a part of him has been ripped away. He might isolate himself for a while, blaming himself for not being able to protect her. But eventually, he would use his pain as motivation to get stronger and fight against whatever caused her death.
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.
One possible plot could be that Naruto sacrifices himself to save the village from a powerful and ancient evil. He uses all his chakra in a last - ditch effort to seal away the threat forever, leaving the village in mourning but also in a state of safety. Another plot might involve Naruto being poisoned by an enemy who has infiltrated the village. Despite the efforts of Sakura and the others, they can't find a cure in time, and Naruto succumbs to the poison. A third idea could be that in a battle against a new, incredibly strong foe, Naruto is overpowered and killed, which then spurs the rest of the shinobi world to come together to avenge his death.
The characters in 'Naruto' would be deeply affected if Naruto dies at birth in fanfiction. Jiraiya would be one of the most affected. He saw Naruto as his protege and the hope for the future. Without Naruto, Jiraiya might lose his enthusiasm for training and spreading his teachings. Tsunade would also be saddened as she had a connection to Naruto through her relationship with Jiraiya and Minato. The Rookie 9, Naruto's friends, would have a completely different dynamic. They might not form as tight a group without Naruto's energetic and positive influence. And the Sannin, as a whole, would have their relationships and legacies altered without Naruto in the picture.
If Naruto dies at birth in fanfiction, it would have a huge impact on the story's positive themes. Naruto was a symbol of perseverance and the ability to overcome one's past and hardships. His death would mean that those themes would be lost. Also, the Akatsuki's plans regarding the Nine - Tails would be disrupted. They might turn their attention to other 'jinchuriki' more quickly. And characters like Iruka, who played an important role in Naruto's life, would have a completely different story arc as they would not have Naruto to mentor and befriend.
Naruto would be devastated. Sasuke is his closest friend and rival. He would likely go into a state of deep mourning at first. He might isolate himself for a while, just trying to come to terms with the loss.