Sirius had no real answer for that, largely because it was almost impossible for him to understand. Sirius could be self absorbed and do rotten, rash things now and then - but he wasn't capable of the kind of mistakes that Regulus had made, not really. "You weren't that free, Reggie," he answered finally, nodding vaguely toward Regulus' arm.
The chocolate biscuits earned another smile and then he sighed. "I don't need you to keep me clean. But fine. You can write Dumbledore, and tell him where to find it. If he wants Harry to find it, he can tell him." And he'd tell James, and they'd watch Harry like sodding hawks so he couldn't run off to find it alone.