That was very sad! The idea of James and Sirius's relationship being so wrong at the time was really cutting, as was the thought that he had to choose Lily even if he would have liked to choose Sirius. And the idea that Azkaban took so many of Sirius's good memories of James and left him with only the bittersweet ones seems very true and was even more effective.
Two questions: you never really mentioned much about James being dead now, and I wonder if that was on purpose, like Sirius is in denial? And I wasn't really sure about how the last paragraph connected, to be honest. I'm sure it's there, but a reference earlier in the story might make it more obvious, since the skull cutting feels a bit abrupt as it is.
Otherwise I think you did a good job with this. I love pairing James/Sirius/Remus up in different ways! Yay! I did James/Remus last year, hee hee. Good luck with your next!
~Gina :)