McGonagall, being a strict but fair teacher, would likely feel a deep sense of outrage. She may offer Sirius a safe place to stay, perhaps in her own quarters or arrange for him to be with a more trustworthy family. She would also probably keep a closer eye on Sirius to ensure his safety in the future.
She might be horrified and immediately take steps to protect Sirius. For example, she could confront the abuser or report it to the proper authorities within the wizarding world.
They might face magical punishment. For example, they could have their magic restricted or be sent to Azkaban if it's a really bad case. McGonagall would not tolerate such behavior and would push for the strictest punishment.
Sirius would likely be extremely angry. He already has a protective streak towards Harry. He might immediately confront those he suspects of the abuse, whether it's the Dursleys or others at Hogwarts. He would probably also try to remove Harry from the situation as quickly as possible.
Well, James is a loyal friend. So, when he discovers Sirius is abused, he would most probably drop everything else he is doing to focus on Sirius. He'd be angry, not just at the abusers but also at himself for not realizing it sooner. He'd make sure Sirius knew that he wasn't alone and that he had someone who would always be there for him. James might also encourage Sirius to learn self - defense magic more intensively so that he could protect himself in case the abusers tried to harm him again.
Remus is likely to be shocked and angry. He might immediately want to protect Sirius. He could confront those who are abusing Sirius or try to find a way to get Sirius out of that situation.
Sirius would probably be very concerned. He might immediately ask her what's wrong and offer her a handkerchief to dry her tears. He has a bit of a wild side, but he also has a big heart, especially when it comes to those he cares about or sees as in need of help.
Well, McGonagall is a very caring and strict character. When she discovers Harry is being abused in fanfiction, she would first ensure Harry's safety. She might offer him a safe haven at Hogwarts all year round if possible. Then, she would start an investigation into the abuse. She'd use her influence and connections in the wizarding world to get to the bottom of it and make sure justice was served for Harry.
Once McGonagall discovers the abuse, the story could take many turns. Harry could be given special training or mentorship by McGonagall to build his self - esteem. The wizarding world might also change some of its policies regarding the protection of children in response to Harry's case. Meanwhile, the relationship between Harry and McGonagall would grow stronger as she becomes a sort of surrogate family for him, and he would rely on her for guidance and support in his journey of recovery from abuse.
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.
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.
James might be shocked at first. He could then feel a sense of protectiveness towards Severus. Maybe he starts to see Severus in a new light and tries to find ways to help him, like keeping an eye on those who abuse Severus.