What a great read. I didn't know what to expect. I don't read much het and very little if no Petunia. But I am so happy I started reading this; it was an intense and very interesting fic.
At '"And that bastard Potter's face, I am informed," Snape added with a heavy sigh. "It is an obscene conjugation."' you had me entirely; from there it only became more bitter-sweet and wonderful. I was impressed with the slow, thorough character development, with the dialogue and your perfect understanding of what drove the two. I particularly liked how you also kept Potter out of the fic, that he was there only mentioned as this annoying appendix to a family which doesn't want him.
As a side character, your Vernon was spot on, precisely as I imagine him.
Great plot overall, and the repeated return to Lily's grave made a great structure. A most enjoyable read.