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 could be that they try to hide Sirius from his family. Another is that they report the abuse to the authorities at Hogwarts.
Sirius' behavior might change. He could become withdrawn or overly jumpy. For instance, he might flinch at sudden movements or loud noises. James, who knows Sirius well, would pick up on these unusual behaviors and start to investigate, which could lead him to discover the 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.
One common theme is loyalty. Sirius is extremely loyal to his friends, the Marauders. So many fanfictions explore this loyalty, like how he stands by them through thick and thin. Another theme is family. His difficult relationship with his family, especially his pure - blood supremacist relatives, is often delved into. For example, stories might show his struggle to break free from their expectations.
Healing is also a major theme. The sister goes through a process of recovering from the psychological and physical wounds of abuse. This can involve making new friends, finding new interests, and learning to trust again, which is often explored in great detail in these fanfictions.
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.