They had been through a lot. Thus, trust was of no issue. Even after the Disaster of '75 did Remus still trust him. For, you see, it was not a matter of trust which Sirius broke – merely a matter of comradeship, morals and simple code of conduct. So while Sirius was regarded as a bastard, he was a trusted one.
It did take some persuasion on his part to convince Remus not to castrate him, but his trustworthiness was never questioned.
After receiving forgiveness, things went back to the abnormality that is the Marauders. Sirius did have to explain why it was, in fact, not a matter of trust to a few people, such as a fiery, redheaded she-demon (his words) however. And his reasoning was such: no-one ever entrusted you not to go off and blag about your mate's lycanthropy and – almost! – kill someone in the process. Therefore, no trust was broken; it was simply something Not Done.
Remus had forgiven him then, on the grounds Sirius HAD NOT GOTTEN THE OWL. The she-demon-on-fire proceeded to think the canines were utterly mad. They offered no arguments. Indeed, it would be fair to say they whole-heartedly encouraged it.
James was of no use either, paralysed in place with an expression of ridiculous (-ly cute – what? Who said that? Definitely not the she-demon…) dumbfoundedness that Evans – Lily Evans! – was talking to him. Granted it was about his idiot friends, but it was a start! Peter just shrugged and said if no-one was disagreeing with her, what was she asking around for?
All in all, Remus trusted Sirius with his life and vice versa. That they had been through a lot just accumulated a great deal more trust. Yeah, they had been through a lot, and they would go through a lot more. Marauders forever?