This story-line seems plausible. Of all the questions JKR hasn't answered, the one of how Lily and James got together is a major one, and yours seems to be the most canon-worthy one that I've read.
I like the "follow-up" at the end, where Lupin and Severus end up facing each other, although I do wonder how involved Sibyll is, and what she knows. But I see that the story is labled, "complete"....
Author's Response: Thanks again for your wonderful reviews, Overhill. Wow, I'm pleased you find this one the 'most canon-worthy' that you've read. That's a nice compliment. I just added Sibyll in there for fun and for her musing about fate. Cheers!