If Snape was a double agent all along, as we later discover, and we know he had been working with Dumbledore, shouldn't we assume he knew the truth about Sirius? Confronting him and Remus and threatening to take them to Dumbledore would only be for show, which means he was setting himself up to be attacked by Harry for no reason. Or does he really not know the details of members of the future Order of the Phoenix?
shareEven Dumbledore believed Sirius was guilty until Harry discovered the truth. Dumbledore has admitted that he too can be and has been wrong.
shareVoldemort was the only one who knew who all his Death Eaters and spies were in case anybody sold them out - which Karkaroff did straight after Voldemort's fall.
Snape likely never saw Wormtail at meetings, who was a spy so was unlikely to be an open Death Eater. He thought, like everyone else, that Sirius was guilty and probably assumed that he'd never seen him at meetings because Voldemort's Death Eaters all knew that only Voldemort knew who they all were.