The Chronicles of King Rolen's Kin, book 3: The Usurper
Rowena Cory Daniells2010

Now a slave, Piro finds herself in the Merofynian Palace where, if her real identity is discovered, she will be executed. Meanwhile, Fyn is desperate to help his brother, Bryen, who is now the uncrowned King. Bryen never sought power but now he finds himself at the centre of a dangerous resistance movement as the people of Rolencia flee vicious invaders. How can Byren defeat the invaders, when half his warriors are women and children, and the other half are untrained boys and old men?