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.
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.
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.
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.