Excellent, just excellent. I normally find Severus/Sirius very difficult to believe, but you do a fine job of making their relationship plausible, developing it slowly and skillfully, bringing them together in logical ways.
I like AU plot -- it covers all the main canon points in believable alternatives, and I particularly enjoyed the way you dispatch in a line or two things that took chapters upon chapters in canon. (Example: the destruction of the Ravenclaw diadem.) I'm not criticizing the canon presentation, since in that version of the story, these incidents were important. But they're not needed in your alternative story; you just need the characters.
And what characters they are! My favorite parts are the conversations between Sirius and Severus: so beautifully IC, so well-structured and paced, just so satisfying overall. I could quote dozens of lines, but these are typically fun:
"One lager? That hardly makes me a drunkard, Snape. But I dare say you could use one more than me. Might loosen you up enough for you to get that hot poker out of your arse." "Alcohol has never helped you to get your head out of yours."
You show us so clearly the adolescents still raging inside the grown men. And a great job with Remus, too -- all of them, really.