A vulnerable yet manipulative Snape. A love once gone. And a brother prepared to take his brother's place. Makes you wonder why. Makes you think of all sorts of scenarios which led to here.
Please continue to write stories other than Snarry. You are fantastic at it.
It's a story where I'm thinking-thinking-thinking after reading it. It doesn't explain itself, and I like that: you can imagine any number of possibilities.
(Plus, of course, submissive Regulus/Sirius and dominant Severus... which is enough to make any story fantastic!)