Honestly, if you ask whether Sirius knew Regulus existed and who he was before the war, the short reality is: absolutely. They were brothers — part of the same Black family tapestry that Sirius eventually tore off the wall — so Sirius was well aware of Regulus as a person and a choice-maker long before anything with Voldemort ramped up.
Reading 'Harry Potter and the Order of the Phoenix' and later 'Harry Potter and the Deathly Hallows' made the family tensions really clear to me. Sirius and Regulus had opposite reactions to their upbringing: Sirius cut himself off and fled the family’s pure-blood fanaticism, while Regulus leaned in, joined the Death Eaters, and became someone Sirius was openly contemptuous of. That contempt is obvious in how little Sirius spoke of him and how bitter he sounded about the family’s values. Crucially, though, Sirius never learned Regulus’s secret redemption — that Regulus tried to undo Voldemort by targeting the locket Horcrux — because that detail only comes out through Kreacher much later. Sirius died believing Regulus was a turncoat toward darkness, not the complicated, regretful figure we learn about afterward, and that tragic ignorance colors a lot of how I feel about both of them.