Lucy is seventeen when she discovers that the women in her family are under a curse by an elf king that causes each of the women to be pregnant at 17, and requires them to perform three impossible tasks. If they can't complete those tasks, they go insane. Lucy has seen the proof of this in her estranged mother, who periodically shows up, crazy and pushing a shopping cart and embarrassing her at school.
What set this book apart from so many young adult novels that I've read is that when Lucy goes to her family to tell them of the curse, they believe her. There's no argument that "there's no such thing as magic or elf kings--you must be crazy", they just believe in her and work with her to help her try to complete the impossible tasks. And they give an excellent explanation that maybe it's true and maybe it's not, but she believes so strongly in it, they need to work with her.
The novel itself is an odd blend--there's a lovely romance with Zach, the boy next door--but it also contains genuine peril, including rape and mental illness. But due to the excellent characterization and the unusually and well-constructed plot, it works very well. This would definitely go on my list of favorite fairy tale/ballad adaptations along with Pamela Dean's Tam Lin and Beauty by Robin McKinley.