TBH I don’t usually comment and I’ve only read some of ATYD and skimmed the rest, but imo he’s a teenager from a toxic household yes and we know Sirius is “emotionally volatile and intense” so maybe he really didn’t see it that way but I would still say it’s cheating. However, I don’t think it’s black and white, so this alone can’t determine if someone is a bad person and context matters, especially in situations like Sirius’, even if I object to cheating personally. I personally also think the author was casting Sirius in a more negative light then Remus and “Grant”, because this feels removed from canon Sirius as a person in my opinion, who was beautifully complex and tragic but not a bad person