Only Themselves To Blame
They think he doesn't know, but very little goes on in Gryffindor Tower that Peter Pettigrew is unaware of. Being a rat has its advantages, though none of them see it that way.
He hears them, the change from hushed laughter to softer, wetter, deeper sounds, grunts and groans he's heard emanating from behind their bedcurtains separately for five years, suddenly together. Words -- fuck, yes, please -- and names -- James. Sirius. Urgent whispers breaking the midnight silences.
He knows why Remus sleeps down in the common room these days, and casts a silencing charm when he doesn't. Knows why Remus is so slow to forgive Sirius when James forgave him the next morning. Which is why Remus is freezing James out as well. Remus is no longer following James's lead, and it's as if a whole new way of thinking has opened before Peter, one where following James Potter is not the only possibility.
Peter knows it was always JamesandSirius, will always be JamesandSirius,
will
He hides the resentment that's started to fester, covers it with laughs and jokes as they try to pretend everything is all right when it will never be all right again, not since Sirius broke their trust, and James forgave him.
When the world turns over again, when JamesandSirius suddenly gives way before JamesandLily, Peter takes grim satisfaction in Sirius's bewilderment, and wonders if Remus feels the same.
During the day, James and Sirius act as if nothing's changed, never knowing they've sown the seeds of their future destruction with thoughtless words and careless actions. But Peter feels the change coming, has felt it for years, and when opportunity knocks, he finally takes it, knowing they'll never believe he could.
And they'll have only themselves to blame.