Information on a book

A while ago I read a book. I want to give my friend a copy but I forgot what it was called and the author. If you know what it is I'd love the help.

The plot is that the main character, Rowan (named that for protection from fairies), has her baby brother kiddnapped and replaced with a changling. She discovers that she's a half fairy and that her parents are actually her aunt and uncle. Her mom is her aunt who got pregnant after a night with at a ballroom dance with a fairy male who disappeared saying he left her a gift. Her brother is actually her cousin.

Feel free to ask questions. But thanks in advance.