The Caged Queen by Kristen Ciccarelli

Once there were two sisters born with a bond so strong that it forged them together forever. When they were angry, mirrors shattered, and when they were happy, flowers bloomed. It was a magic they cherished—until the day a terrible accident took Essie’s life and trapped her soul in this world.

Dax—the heir to Firgaard’s throne—was responsible for the accident. Roa swore to hate him forever. But eight years later he returned, begging for her help. He was determined to dethrone his cruel father, under whose oppressive reign Roa’s people had suffered. Roa made him a deal: she’d give him the army he needed if he made her queen.

Together with Dax and his sister, Asha, Roa and her people waged war and deposed a tyrant. But now Asha is on the run, hiding from the price on her head. And Roa is an outlander queen, far from home and married to her enemy. Worst of all: Dax’s promises go unfulfilled. Roa’s people continue to suffer.

Then a chance to right every wrong arises—an opportunity for Roa to rid herself of this enemy king and rescue her beloved sister. During the Reliquishing, when the spirits of the dead are said to return, Roa can reclaim her sister for good.

All she has to do is kill the king.