Oh, oh, oh! I completely LOVE this! And I don't even like Sirius! But you made me love him here, the stupid git. It's such a wonderful emotional siege that he lays on Severus. Weirdly, this story reminded me of The Breakfast Club - in which people are stuck together and they have this intense effect on each other.
I adore Wormtail's sense of smell that can detect subtle combinations of emotions. Also, did I mention that the sex is totally hot? Oh, and the ball! The masks spelled to show the emotions of the face beneath, and the glamours that change the rest of your looks! You did a fabulous job of creating the Manor and the elegant, libertinious (is that a word?) party.
The way you use masks here! It's just gorgeous that Sirius, who already loves Severus, can only fall in love with him when the "Snapeness" is hidden, so that the masks are what reveal the truth.
I love your Sirius. He's such a pathetic and lovable idiot with his emotions all over the place, his foolish pride and ADD and lack of planning. A dog, indeed.
I'm going to come back and read this again, I'm sure.