"I don't know, I'm considering it." He was just being contrary because he could at this point. He didn't know what point he was trying to make, other than the fact that everything felt wrong. That he felt wrong, and didn't feel like he was in a place to be able to fix anything no matter what Regulus was trying to convince him of in that moment
He sighed as Regulus spoke, about that time in their lives when they had been on the same side. Back when they'd been all the other had in a world that seemed against them from far too many angles. Before Hogwarts and everything else. It felt like it was from another life entirely, Sirius could hardly remember that long ago, not after all the pain, all the loss, and everything else that he'd gone through since then. And then Regulus made his next confession.
"You joined that monster just to spite me?" He felt like he'd been cursed all over again, that his brother could have been so petty, so unwilling to actually talk to him, to hear it from Sirius himself what he actually felt, or to even tell Sirius what he thought of him before he let himself get wrapped up in all of that. "Dammit Reg, I never fucking forgot about you - I don't know how many ways I can say it to make you hear me. You've always been my brother."