The Uncrowned King by Rowena Cory Daniells

Thirteen-year-old Piro watches powerless as her father's enemies march on his castle. A traitor whispers poison in the King's ear, undermining his trust in her brother, Byren. Determined to prove his loyalty, Byren races across the path of the advancing army, towards the Abbey. Somehow, he must get there in time to convince the Abbot to send his warriors to defend the castle.
Meanwhile, the youngest of King Rolen's sons, Fyn, has barely begun his training as an Abbey mystic, but he wakes in a cold sweat, haunted by dreams of betrayal...
Includes bonus novella The King's Man
Garzik, younger son of Lord Dovecote, has been captured and sent to Merofynia as a prize of war. Now, he must set things right before he can return home. Turning his misfortune into opportunity, Garzik resolves to spy for the rightful king, Byren, who yet thinks him slain at Dovecote. With fortune on his side, he may learn something that could change the path of the war, for Garzik is and always will be the king's man.