I do wish we knew, however, what made Sirius suspect Remus. Was it war-driven paranoia? Was Remus doing the platonic equivalent of what he does to Tonks, pushing them away? If it was the latter, I can understand Sirius misinterpreting it as something else. And did James and Lily suspect him, or just Sirius?
Perhaps Sirius and Remus were never the biggest buddies everrrr. I can see them having a sort of triangle, with James as the connecting middle. From what I can remember from Snape's Worst Memory (euggghhh) and from what McGonagall had said, James was always the ringleader, ánd he was Head Boy and everything. I can see James being a little more sensible and mature IF NEED BE, while Sirius was just all fun all the time. Remus would connect better with James, if that were the case, because they'd have something in common.
And that would explain why Sirius would suspect Remus, if they hadn't always seen eye to eye all the time, or thought the other was a bit weird to begin with ("he likes books/messing around?? Weirdo").

Of course, that totally doesn't gel with Remus/Sirius but eh. x)