The Singing Sisters (Princess Minna 7) by Kirsty Applebaum (Paperback)

"Princess Minna is visited by Princess Sky-Blue and her eleven sisters. They're going to have so much fun! But when the train arrives, all of Sky-Blue's sisters have been turned into nightingales by a wicked witch! Princess Minna is going to need all her skills and all her bravery to put this one right"--Amazon.