Wormtail watched the two in fascination, they couldn't see him but he could definitely see them, smell them, hear them. . .

His little rat body quivered in excitement when James bent down to take Sirius in his mouth, he twitched his whiskers wishing he could have watched in human form, his little rat cock was firm and at the ready.

When Sirius moaned Wormtail closed his eyes, imagining it was him James was pleasuring, him James would be pounding into later. . .