One possible reason could be that his sisters have some special abilities or qualities that overshadow Naruto in his family's eyes. Maybe they are prodigies in ninja skills from a young age, and the family focuses all their attention on them. Another reason might be that Naruto has some sort of accident or incident in his childhood that makes the family see him as a disappointment, so they start to neglect him.
Perhaps the family blames Naruto for something that isn't really his fault. Maybe there was an attack on the family, and Naruto was the only one present at the time, even though he was too young to do anything. The family, in their grief and confusion, wrongly holds Naruto responsible and starts to neglect him. Or it could be that Naruto has a different personality from his family. His more rambunctious and independent nature might not be liked by his family who prefer more docile and traditional behavior like his sisters, leading to his neglect.
One possible reason could be jealousy. If Naruto has the rare Mokuton ability, his family might be jealous of his potential power and neglect him out of spite.
Maybe there are secrets in the family. For example, his parents might be involved in some dangerous and secretive missions that require them to keep their distance from Naruto to protect him, but Naruto misinterprets it as neglect.
One possible reason could be that they are too focused on their own problems or duties. Maybe they are involved in some important shinobi business that takes up all their time and attention, causing them to overlook Naruto.
He might focus on his ninja training more. By pouring his energy into becoming stronger, he can distract himself from the neglect at home. Also, he could make friends outside his family, like Sasuke or Sakura, who would provide him with the support and friendship he lacks at home.
It could also be due to some family curse or prophecy. In the Naruto universe, there are often strange powers and curses. Maybe there was a prophecy that said Naruto's family had to keep their distance from him to protect him from some greater evil. So, they chose to neglect him in order to keep him safe, even if it was a difficult choice.
One possible reason could be jealousy. Maybe Naruto has some special abilities or a prophecy surrounding him that makes his family members, who might be less favored, jealous. For example, if there's a prophecy that Naruto will be the most powerful ninja in the village, his siblings or parents might envy that future for him and start to hate him out of their own insecurities.
One possible reason could be his own missions and responsibilities. Jiraiya is often away on important spy missions or gathering information for the Hidden Leaf. He might be so caught up in these tasks that he unintentionally neglects Naruto.
Another reason might be related to family politics. In some complex family structures in the Naruto universe, there could be different factions. Naruto might be on the wrong side of the power struggle within the family. His views or actions could be seen as a threat to the traditional power structure of the family, so they hate him.
In some fanfics, it could be related to a prophecy. His family might believe that Naruto is going to bring about a great disaster according to a misinterpreted prophecy. So, they turn on him in an attempt to prevent the prophecy from coming true, even though they are wrong in their understanding.
Sally could be in a new relationship or have new friends that are demanding a lot of her time. As a result, she may not have as much time or energy to devote to Percy, which unfortunately comes across as neglect in the fanfiction.