Oh wow, this is wonderful! I love post-Azkaban, fucked-up Sirius, and you wrote him so well.
He'd handled that conversation well--he sometimes felt like he was only impersonating an adult, but this time had been all right.
That line right there is just perfect, sums up the whole story so well. He's so damaged and crazy but also in perfectly understandable ways, for his circumstances. And I loved the Snape cameo! Great way to work in the hatred between those two. Such a good story!