Epic Fantasy Book Cover: Love and Battle

A fantasy book on a young woman that becomes a fighter and falls in love with a dark prince. She lives in a realm where all families must have four children: 1 to serve the family, 1 to serve politics, 1 to serve education, and 1 to serve the army. The girl is the 5th child and can choose her own destiny. But as she is about to pick a path, the brother of the queen, the dark prince, chooses her to follow him as one of his fighters.