like, "golden children" of abusive parents unquestionably experience abuse, even if it's not physical abuse. but, to put that onto another abused child is insanely fucked up (1. People, especially children, can't control or stop other people's abusive actions, 2. Even if they could, children aren't responsible for their parents actions. 3. and believing that you are responsible for your parent/abusers actions, and for "protecting" siblings from the abusers is exactly what abusers want you to think, so that you don't leave. That guilt, is literally part of the abuse)
Now, reg can feel whatever feelings he feels about Sirius leaving. He can feel betrayed and like he was abandoned to his parents abuse (I mean, we have no canonical evidence that he did feel that way). Interpersonal relationships are complicated. Being a kid, especially one who isn't having their needs met, is complicated. I know lots of people with shitty to abusive parents who feel anger and resentment towards their siblings for not doing more to help and care for them in those situations. But regardless of how reg feels, that doesn't change the fact that Wally and Orion's mistreatment (of either of their sons) was not Sirius' fault. He's not a selfish or bad person for escaping an abusive situation. No one is. It might have irreparably damaged his and reg's relationship, but at the end of the day, that is still on their parents, and Sirius isn't in the wrong for getting the hell out of that situation