Do you recognize this YA dragon book?

It's been driving me crazy. I can't remember the title. There's a princess, and a boy named Owen who takes her place at school because somehow they're identical, and then the girl goes off on a quest to prove that she's the keeper of the light or something? The keeper of the light needed to have a birthmark in the shape of a dove. I think it's a trilogy, and they used "joy juice" instead of alcohol. Oh, and there was something called he bloodpox and there was a small cute girl infected with it. The cover has a white dragon with a girl in a purple dress (I think). In the end, Owen turned out to be her long-lost brother and they were the keeper of the light together. Does this ring a bell for anyone?