Ember Queen by Laura Sebastian

Princess Theodosia was a prisoner in her own country for a decade. Renamed the Ash Princess, she endured relentless abuse and ridicule from the Kaiser and his court. But though she wore a crown of ashes, there is fire in Theo’s blood. As the rightful heir to the Astrean crown, it runs in her veins. And if she learned nothing else from her mother, she learned that a Queen never cowers.

Now free, with a misfit army of rebels to back her, Theo must liberate her enslaved people and face a terrifying new enemy: the new Kaiserin. Imbued with a magic no one understands, the Kaiserin is determined to burn down anyone and everything in her way.

The Kaiserin’s strange power is growing stronger, and with Prinz Søren as her hostage, there is more at stake than ever. Theo must learn to embrace her own power if she has any hope of standing against the girl she once called her heart’s sister.