In many fanfictions where Percy Jackson is Nico's brother, it creates a whole new dynamic in the story. It often leads to different interactions with other characters. For example, their relationship with Annabeth might change. Percy could be more protective of Nico, and Nico might feel more secure having Percy as a brother. It also gives room for exploring their family history in new ways, like how they deal with their demigod powers together and support each other in the face of various challenges in the demigod world.
His relationship with Annabeth would definitely be affected. She might be a bit wary at first, since Nyx is such an unknown and powerful entity. But she would also be there to support Percy as he figures things out.
Among the demigods at Camp Half - Blood, some of the more romantic - minded ones might flock to Percy, thinking he can give them advice on love since he's related to Aphrodite. But Percy would be really out of his element here. And his relationship with the more 'tough' or battle - focused demigods might be strained, as they might see his Aphrodite connection as a sign of weakness or something that doesn't fit with their warrior - like image of him.
Well, one possibility could be that Percy discovers his connection to Nyx and has to deal with the powers and responsibilities that come with it. Maybe he has to face some ancient enemies of Nyx who are now after him because of their relation.
One possible plot could be Percy discovering his connection to Achilles while on a new quest. Maybe he has to face a powerful enemy that only his newfound link to Achilles can help him defeat. For example, he might gain some of Achilles' combat skills or invulnerability in certain situations.
The fanfiction could revolve around the origin of Percy's brother's space powers. It might be that he was experimented on by some dark force in the past which gave him these powers. Percy and his friends then have to find a way to reverse or control these powers before they go out of hand. The brother could accidentally be creating black holes or warping the demigod world with his unstable powers, and Percy has to use his own skills and the help of the gods to save the day.
There could be. Fanfiction allows for a great deal of creative freedom. This particular concept could lead to some really exciting stories. For example, it could explore how Percy's connection to the Greek gods and Harry's connection to the wizarding world would co - exist within a family dynamic. It might also look at how their different skills and upbringings would affect their relationship as brothers. Additionally, it could introduce new characters or villains that are a combination of the magical and mythological elements from both series.
Perhaps she's a sister who was thought to be dead but actually survived. She could have been corrupted by the experiences she had while being 'dead'. When she returns, she blames Percy for not saving her properly. This false blame leads her to betray Percy. She could start spreading rumors about Percy being a coward or a bad leader among the demigod community.
Fanfiction often takes creative liberties. In this case, it might be that the author wanted to explore new power dynamics and character interactions. If Percy was Zeus' brother, it would completely change the power balance in the world of the series. Percy's connection to the sea and Zeus' control over the sky could lead to all sorts of interesting plotlines, like joint quests or power struggles between the two 'brothers'.
Percy might become more confident in his abilities. Knowing he is related to Achilles, a great hero, could give him a boost. He may also become more strategic in combat, learning from Achilles' military tactics.