Regulus wasn't a real brother to Sirius. Blood doesn't mean anything, I know that better than some! I think James acted more like a brother. But, if your asking biologically then Regulus. But, I never see them as brothers, seeing as Regulus stood by while his parents bullied Sirius.

^ I am just asking who is Sirius’s brother who would love him as an brother would do seeing James is younger than Sirius

Biologically, Regulus. But I think James was more of a brother to Sirius than Regulus.

Given that both Sirius and Regulus Black share the same last name, isn't the answer to this poll downright obvious??

Unless you mean relationship-wise rather than biologically, @TheBandit025Nega, then that's a different story

@Kidsayshi that's true. And to paraphrase a quote used by a YouTuber, "Family isn't all about blood and genes. Some parents out there are super abusive and violent and their kids don't call them 'family'. There are friends, teachers, and other people, all with big hearts, whom we see as family." Your comment perfectly matches my POV! ;)