Another possibility is that there could be a new, original antagonist created specifically for the fanfiction. This character could be someone who has a grudge against Naruto's family and wants to destroy their bond. They might try to turn Naruto against his sister or vice versa, creating internal conflict within the family while also posing an external threat.
The main antagonists could be the traditional Naruto villains like Orochimaru or the Akatsuki members. They might target Naruto's sister as a way to get to Naruto, seeing her as a weakness or a way to manipulate him.
The Akatsuki could be the main antagonists. Since Naruto is an important target for them, and if Kurenai is now his sister, they might see her as an obstacle in their way. They could try to use their various powers and strategies to either capture Naruto or cause harm to both him and Kurenai.
It could be some rogue ninjas who are after Lemon for some reason. Maybe they think she has a special power that they can use for their evil plans.
There could be other rogue ninja groups who see Naruto's sister as a threat or a valuable asset. They might try to either eliminate her or force her to join them. These groups would be antagonists as they oppose her in her new rogue lifestyle and she has to constantly fight or outwit them.
Well, the main antagonists could be a group of rogue ninja who are hired by a powerful nation to take down Naruto. Since he is unkillable, they would have to use more underhanded tactics like trying to isolate him from his friends and allies. Or perhaps there is a dark force that is awakened by Naruto's unkillable state, thinking that he is a threat to the balance of the ninja world. This dark force could then become the main antagonist, trying to destroy Naruto at all costs.
One possible plotline could be that Naruto's sister secretly helps him during his difficult times in the village. Maybe she provides him with extra training resources or helps him outwit the bullies without him knowing it's her at first.
It could be a new group of rogue ninjas who oppose the ideals of the Hidden Leaf Village. They might target Kakashi's daughter because of her connection to Kakashi and Naruto, seeing her as a symbol of the village's new generation of strength.
It could be anyone really. It might be some of the traditional Naruto villains who find a way to cross over into Ruby's world. Or perhaps it could be new villains created specifically for this fanfiction that are a threat to both Naruto and Ruby.
In this fanfiction, perhaps Naruto's sister has some special power. Naruto loves her not only because she's family but also because he sees the good in her power. He might teach her how to control it and use it for good. They could go on adventures together, with Naruto always being the over - protective brother and making sure she doesn't get into too much trouble.
Yes, there are some. You can check out fanfiction websites like FanFiction.net or Archive of Our Own. Just search for 'Naruto loves his sister' in the search bar, and you'll find a variety of stories. However, keep in mind that some might deviate a lot from the original Naruto theme and character relationships.
One possible antagonist could be a jealous rival among the pirate gangs. This person might envy Harry Hook's sister's skills or popularity. Additionally, a corrupt official in the Isle of the Lost who wants to keep the status quo and sees her as a troublemaker could be an antagonist. Also, if there are any ancient curses or dark spirits on the isle, they could turn against her and try to stop her from achieving her goals.