Princess Power #5: The Stubbornly Secretive Servant by Suzanne Williams

The princesses are having a ball visiting Lysandra's sister, Gabriella, and brother-in-law, Jerome. And they can't wait for Jerome's handsome brother, Prince Jonathon, to join them. But when he never arrives, everyone panics. Lysandra and her friends must find the missing prince—without the help of his stubborn servant, Thomas, who's not saying a word!