"I know." And Remus does know, because Sirius had given him an elaborate apology letter, and then apology speech, and at the end of the day Remus knew Sirius had never meant for anyone to be killed, even Snape. Remus had forgiven Sirius for his mistake, especially since no real harm had come to it and he had realised his folly in goading Snape into following him the night of the full, and James had been able to stop it all and save the boy.