Sirius might be completely devastated. He could go into a state of shock at first, not believing the news. Then, he might be filled with guilt for not being there for his brother, and anger towards the situation that led to Regulus' death.
Sirius' reaction would be intense. Initially, he'd likely be in denial, refusing to accept that his younger brother was gone. As the reality sank in, he would experience a deep sense of sorrow. He may isolate himself from others as he tries to come to terms with his loss. He might also start to question his own actions and decisions in relation to his brother. For example, if he had left home and left Regulus behind, he'd wonder if he could have done something different to prevent this tragic outcome.
Well, Sirius would probably have a complex reaction. On one hand, he'd be heartbroken. He may start to recall all the memories he had with Regulus, both the good and the bad. He might also feel a sense of regret for any unresolved issues between them. And he could become determined to find out the exact circumstances of Regulus' death, perhaps seeking revenge if it was due to foul play.
There are several potential themes. Forgiveness could be a theme as Sirius may have to forgive himself for any wrongs he believes he did to Regulus. Another theme is the power of love within a family. Even though they had their differences, there was still love between them. The theme of mortality is also relevant as Sirius is faced with the finality of his brother's death. This can make him question his own mortality and the purpose of life. Also, the theme of discovery is possible as he tries to find out more about what happened to Regulus, which could lead to new revelations about their family or the wizarding world in general.
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.
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.
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 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.
Harry might be initially shocked. He could feel a mix of sadness and a sense of responsibility. After all, Sirius was like a father figure to him.
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.
He might be initially shocked. Snape has deep and complex feelings for Lily. His reaction could range from a moment of stunned silence as he processes the news. He may also feel a pang of jealousy towards the father, whoever that is. Given his love for Lily, he'd probably be very protective in his own way, despite the situation being complicated by their past and his past actions and associations.
Snape might be secretly pleased at first, seeing as he has no love for Sirius. But then, being a teacher, he'd probably feel obliged to report it. He also might use this situation to his advantage against Sirius in the wizarding politics within Hogwarts.
He might be shocked at first. Draco, despite his past rivalries, could feel a sense of protectiveness. He might initially struggle with his own beliefs and prejudices but then decides to help Hermione in secret. He could use his family's resources to make sure she is safe from further harm.