Disclaimer: Much to my regret, I don't own Harry Potter, nor do I make any money off him. Oh well... one can only dream.
Regulus never says anything to Sirius. They always pass each other, in between classes and meals and quidditch, and he sees his older brother laughing with his friends. He sees his replacements, and sometimes a funny feeling clenches around his heart. It's a stuffy, stifling feeling that leaves him short of breath and wanting to talk to his big brother again. Once, Reg even opened his mouth as Sirius passed by, a plea on the tip of his tongue, but then Potter sneered at him (filthy Gryffindor bastard) and Reg lost his nerve, slamming his mouth closed forever.
Severus never says anything about Reg's obsession. But he sees it.
Regulus' eyes are constantly on his older brother, begging for forgiveness, and much as Severus hates the mongrel (filthy Gryffindor bastard) he can't quite fault Reg for his yearning. After all, Severus knows how much it hurts when your family turns on you, doesn't he. He knows how it tears you apart worse than you'd ever thought possible, and he hates to see Reg hurt, hates it even more that he hates Potter. So when he sees Reg watching Black and longing to be brothers again, he stills his sharp tongue and never opens his mouth, because that would only hurt Regulus more.
They never say anything, after. They don't need to.
It's already there, racing through their blood, from the press of bodies against each other, from the grasp of hands that don't let go, from Regulus nestling his head under Severus' chin and the way that Severus holds his lover closer and the way they close their eyes and try to forget, forget, forget everything they've been through (they won't touch us again, the filthy bastards filthy bastards filthyfuckingbastards).
So they sit quietly and cuddle and dream and pray that soon it won't be so bad, because He promised that things would be better, and unlike all the others, He hasn't broken any promises.
Yet.