If you wrote this for a Snape/Lupin-specific Fest - not a normal monthly challenge, but a special fest - and have been posting it in a Snape/Lupin-specific community and if you really do plan to end it with Snape/Black, it's my opinion that you need to make that very clear in your header information because I imagine there are some readers who would like to know what to expect and what to avoid before getting into it.
Given the supposed reason you wrote the story and where you're posting it, readers are going to have certain expectations, and I think some of them might want to be warned that it doesn't end with Snape/Lupin and will, in fact, be Snape/Black, especially since not all RL/SS fans are fond of Sirius.
I haven't read the story; I'm basing my observation on what I see in the comments here, so if I'm wrong in my assessment, I apologize in advance.