Sirius could see where Remus' mind was going as he thought over what James had said, and what James said made sense. It was the smart thing to do, to make these sort of arrangements for when something happened, just in case a safety net to keep the Order stable and functioning when the unthinkable happened. But he also knew it was out of character for Dumbledore to be the type to plan for his death - he was Dumbledore for Merlin's sake.
But that was the point Sirius always came back to when he was thinking about the why's involved in all of this. It was nearly impossible to make sense of some of the things the man did, but everything had a purpose, Sirius could trust that much - he'd started questioning some of the things Dumbledore said, but he still trusted him. And that's where he was now, he'd be running around in circles forever if he tried to make sense of everything Dumbledore did.
"I'm sure some of your questions will at least be answered at the meeting," Though they'd probably end up with ten more for their trouble.