Sirius stopped walking long enough to put an arm around him. He was grateful Peter would still open up to him about this stuff, after Sirius' literal meltdown the last time they talked.
"I'm really sorry, for all of it. For her, because she was a really good person, and for you, because you deserved that."
He sighed, then shrugged.
"I mean, do I wish it was literally anyone but Regulus? Yes. Can I change that? No. So, what's to talk about?" The only thing talking would do was rip open the feelings he'd just managed to get fully locked away. He'd already freaked out at Peter, he wasn't going to do it again.