Child of Faerie, Child of Earth by Josepha Sherman.
I read this book years and years ago, and when I found it in the library, I had to get it. I think it’s based on a fairy tale found in one of Andrew Lang’s color books. A beautiful young woman is hated by her new stepmother. The stepmother is very ugly (her nickname is Toad) and she practices the Black Arts. The protagonist of the story would be in a lot of trouble except the Queen of Fairies’ son has fallen in love with her and vows to protect her.
There is a lot of subtext of the adolescent pull between following parental and social guidelines and discovering your own power. It is extremely charming, although less compelling now that I’m at least twenty years older now.