If Sirius had a child, realistically speaking he would not be a good father. With Harry sure, he knows how to be on fun uncle duty, but when it comes to being a dad he’s really not that great. He has his temper and he is far too proud to go to therapy. He would never actually hurt his kid, that’s a line he promised to never cross. He tries so hard to be better than his own parents but ultimately he falls short, leading to a complicated relationship with his kid.
However I do think that child would somehow get along excellently with Regulus. He would see Sirius favouring Harry, preferring a Potter over his own blood once again and he’d step up for his niece/nephew. Not that he’d be perfect, but Regulus is far more composed than Sirius ever was. Perhaps unstable in other ways, yes, but clever enough to be some sort of guiding figure to a child.