hc that Sirius is super protective over people he loves. like he's insanely protective. he goes into guard dog mode. if one person is even slightly rude to people he calls his, bam he lashes out in pure fury.
this is rooted in the way he always had to protect Regulus from his parents when they were kids. dysfunctional families often convince siblings that they have some sort of responsibility over each other. so its actually natural for both Regulus and Sirius to develop a strong sense of protectiveness over their loved ones. Sirius shows it loudly, lashing out at people who even breath the wrong way at his friends. while Regulus' protectiveness is more calculative and collected when he acts, but just as (if not more) dangerous