I'm sorry it took so long for me to reply. I totally forgot that I hadn't.
Thank you! I'm glad you like it. I don't really know how Sirius ended up in this relationship, I was originally trying to go someplace else with the fic but the story decided it wanted to be written like this.
In my mind, it would be perfectly within Snape to do something like this for reasons that, at least to him, make sense...