The Thirteenth Princess by Nina Clare

The true story of the twelve dancing princesses has now been recorded…Thirteen sisters have grown up hidden away by their Uncle, who rules the kingdom by proxy. When the eldest sister marries, her husband will take the throne. But the king has no intention of giving up his crown. Navigating an underground world, breaking an enchantment, and enduring the prospect of a forced marriage, these are the trials the thirteenth princess must endure to see her sisters and their kingdom free…