After rereading your story (can't get too much of a good thing), I have a question: if Lily is not related to Petunia, how can Dumbledore's blood wards work?
That's just a small glitch though. I agree with other reviewers that this one-shot would make a great jumping off point for a full length fic.
BTW, I failed to mention in my other review that Snape's ruminations on how abuse and neglect can make a child a perfect candidate for Slytherin was spot-on. Are you a psychologist by any chance?