Remus knew this reaction. Sirius was confused, hurt, angry... so he'd focus on angry. He took a deep breath, hoping to change his personal perspective a bit, to maybe help Sirius understand this to avoid having to deal with it himself. But then, how could he possibly help Sirius understand it when he himself had no idea?
"Sirius... please, calm down." he said quietly, his skin still a rather sickly colour. "Perhaps before you loose your temper, we should find out exactly what happened, because I swear to you, I have no possible reasoning about how..." he shook his head, turning to James for a moment.
"Is it possible that this is just a joke? Someone taking us all for a ride?" he asked, inhaling sharply as he tried to restore his breathing to normal.