[Leisha]

The entire Order couldn't have known, or else they'd all have immediately realised it was the Secret Keeper who blabbed.

I'm thinking it was like this: Sirius asked Wormtail to be the Secret Keeper instead of him. Wormtail was made the Keeper. No one else knew that they'd swapped, not even Dumbledore. Wormtail told the secret to Voldemort. Voldemort killed the Potters and blew up the house. As soon as the building collapsed, the spell collapsed too, which meant everybody in the Order suddenly just "knew" the Potters' location somehow.