Regulus knew his brother was hardly agreeing to not kill Peter, if his brother agreed to something he made that emphatically known, no slight nods. But Regulus let it go, he wasn't sure he himself could really go into what a fractured soul felt like, and he hoped Peter never showed up in this crazy place if only for Sirius's sake. He nodded slightly in acquiescence to the subject change.
Regulus, trepidous at first when Sirius claimed he could make his life hell, barked a laugh at the offer, "I wouldn't know where to begin, why don't you just tell me something you think I don't know, save us time while I try to think of something to ask."