Sirius didn't know what to do now that suddenly Dumbledore was there with them as well - he'd still had more questions for Regulus when their former Headmaster had shown up. And so he sat quietly, letting Dumbledore talk to Regulus and Remus. He would have preferred not having his brother there - something about having all of them in the same room, especially given their conversation, made him a little twitchy.
When Dumbledore turned towards him and James, Sirius kept a hand on James - for one of the first times in hide life feeling uncertain when faced with the man. He shrugged when Dumbledore asked if they'd heard about his plans. "I'm not surprised you want to make some changes." He answered instead, it made sense even apart from these new developments with Remus.
"Of course you should be there, Moony." Sirius insisted despite his friend's attempt at pushing it away - Remus was a part of this, and he should be a part of any new ranks that Dumbledore wanted to make as well, they'd been there since they finished with school and Sirius wasn't about to let Remus be left out now for whatever reason Dumbledore might have tried to come up with, though hearing that Dumbledore also agreed that Remus should be there went a long way to calming his nerves.