The Steel Princess. The Sovereign Blades, #1

Born a princess. Made an assassin. Fated to change the world. Skye was never meant to rule. As the third child of Eskeleth's king, she's instead spent years learning the skills of war and death, to protect her country and the older sister who will one day be queen. Except disaster has befallen the royal family, leaving Skye next in line for the throne. For the sake of her people, she must return to a homeland plagued by ghosts - and bordered by a rapacious Empire - to fight for her crown against a Council that will do anything to keep her from it. And if Skye is to rule, she must overcome an ancient prophecy, one that promises Eskeleth's end in blood and fire.
To save her kingdom, she will need to be both princess and assassin, or everything she's ever known will be destroyed.