Birthright by Missouri Vaun

When her spies bring news that a swordswoman imprisoned in a neighboring kingdom bears the Royal mark, Princess Kathryn sets out to rescue Aiden, the woman she's sure is the true heir to the Belstaff throne and the solution to a tyrant's campaign to overrun her small queendom of Olmstead. That solution, however, is more of a problem. Too footloose for responsibility and distrustful of Kathryn's selfless moral code, Aiden is a resistant heir. Her only interests are freedom and adventure. Despite their differences, Kathryn and Aiden discover common ground and a growing attraction as they set out on their mission to defeat the ruthless rogue ruler of Belstaff. The true test lies ahead, when they find they must free their hearts to finally liberate their queendoms.