Well, a likely plot development is that they start planning to help Sirius break free from his family's control. They might also try to gather evidence of the abuse to expose his family. And perhaps they would form a sort of protection squad around Sirius, with each of them taking turns to make sure he's safe. Maybe James would even offer Sirius to stay at his place over the holidays to keep him away from his abusers. Remus could use his knowledge of the wizarding world's secrets to find a more permanent solution for Sirius. And Peter, while not as strong - willed as the others, could contribute by being an extra set of eyes and ears to detect any threats to Sirius.
One possible plot development could be that they try to hide Sirius from his family. Another is that they report the abuse to the authorities at Hogwarts.
There could be a plot development where Remus and Sirius go on a journey to find a new place to start over, away from the abuser. Another development could be that Remus uses his intelligence to outwit the abuser legally. The fanfiction could also show how Remus helps Sirius regain his confidence and self - esteem. As Sirius heals, they might start to explore their relationship in a deeper way, whether it's a stronger friendship or something more romantic. There could also be a sub - plot where other characters get involved in different ways, some trying to help and some causing more trouble.
They are likely to be horrified. James, for example, would probably be extremely angry that his friend had to endure such a thing. He might immediately vow to protect Sirius from any further harm. Remus, being more empathetic, would be there to comfort Sirius and offer his support in a more quiet and understanding way. Peter, though not as strong - willed as the others, would still be shocked and follow the lead of James and Remus in trying to make Sirius feel better.
One common theme is friendship and loyalty. The Marauders' unwavering support for Sirius showcases the strength of their bond. Another theme could be healing. Sirius has to go through a process of emotional healing, and the fanfiction might explore how he does that with the help of his friends. Also, there might be a theme of justice, especially if the abuser is still around or if there are people who need to be held accountable for what happened to Sirius.
They would be shocked and angry. James might immediately want to confront Sirius' family. Remus would be empathetic and try to comfort Sirius. Peter, though a bit cowardly, would also show concern.
One possible plot development is that the gods gather to hold a trial for the abuser. They would bring in witnesses, and Percy would have to testify about what happened to him. This would be a very emotional and intense scene.
There could be a plot development where the Weasleys' discovery of Harry's abuse leads to a rift in the wizarding community. Some might not believe Harry was abused or might try to cover it up. The Weasleys would then have to fight against this disbelief and secrecy. They could also form alliances with other families or characters who believe Harry. In the meantime, Harry might start to develop new magical abilities as a result of the love and support he gets from the Weasleys. This could be a turning point in his growth as a wizard, as he learns to use his magic in a more positive and confident way.
One plot development could be Lucius using this information to try and manipulate Harry. He might offer Harry a way out of his abusive situation in exchange for Harry's loyalty or some other form of service. For example, he could ask Harry to spy on Dumbledore for him.
One possible plot development could be Nico seeking justice for Percy. He might start to investigate the abuse on his own and gather evidence against the abuser.
Perhaps there could be a plot where Annabeth discovers that the abuse has given Percy some hidden powers or a different perspective on his demigod abilities. So she encourages him to explore and master these new aspects of himself. Or, it could be that the news of Percy's abuse spreads among their friends, and they all come together to form a support system for him, which in turn strengthens their group as a whole.
After the discovery, a possible plot development could be that the champion takes Harry away to a secret location to keep him safe. From there, they might try to heal Harry's emotional wounds. As they do this, they could encounter various obstacles, like the abuser trying to find them or the wizarding authorities not believing their story. The champion might then have to prove Harry's situation, which could involve finding witnesses or evidence. Another development could be that the champion's actions draw the attention of a powerful and mysterious figure in the wizarding world, who either helps or hinders their efforts to protect Harry. This could lead to a whole new set of challenges and adventures in the fanfiction.