The Thirteenth Princess by Diane Zahler

Zita is not an ordinary servant girl—she's the thirteenth daughter of a king who wanted only sons. When she was born, Zita's father banished her to the servants' quarters to work in the kitchens, where she can only communicate with her royal sisters in secret.

Then, after Zita's twelfth birthday, the princesses all fall mysteriously ill. The only clue is their strangely worn and tattered shoes. With the help of her friends—Breckin the stable boy, Babette the witch, and Milek the soldier—Zita follows her bewitched sisters into a magical world of endless dancing and dreams. But something more sinister is afoot—and unless Zita and her friends can break the curse, the twelve princesses will surely dance to their deaths.

A classic fairy tale with a bold twist, The Thirteenth Princess tells the unforgettable story of a magical castle, true love, spellbound princesses—and the young girl determined to save them all.