Of course Dumbledore would find out, especially now that he was back at the castle - but that should be something Dumbledore wanted, he knew how much Dumbledore had wanted Regulus to join them, though Sirius could see well enough that was something so unlikely to happen, especially now with this news. And he had to wonder what had been going through the Headmaster's mind when he'd tried to force his hand with Remus.
"Do you believe Snape?" Sirius asked his brother, because this was going to be the important part. Snape could clam the sky was purple for all he cared but what really mattered was if Regulus believed him, if he trusted that his friend was telling the truth and hadn't been a part of this.
He swore when Remus started telling him about organizational changes, that just meant bringing in more people, people Dumbledore didn't trust and when you weren't trusted you were expendable. And if Remus wasn't trusted .... "He has to know it would be stupid not to have you though, he wouldn't -" He shook his head, Remus was not just someone you threw away, what had happened at things were so changed. It used to be so easy to follow orders, to be a good soldier - now suddenly things didn't seem so easy.