I'm sorry it took so long for me to reply. I totally forgot that I hadn't. Thank you and I'm glad you liked it.
I never really came up with a reason why Sirius took his place. Originally it was going to be a lot more angsty/dark where Severus was polyjuicing Sirius into his brother without his knowledge but it changed during the writing process so that Sirius ended up knowing but I still haven't completely figured out why he would. With the animosity between the two, there needs to be a really good reason for him trusting Severus to the extent that would be needed in this type of relationship. It's an interesting dilemma though.