The Shattering (Guardians of Ga'Hoole, Book 5) by Kathryn Lasky
Fifth title in a mythic adventure series in which the heroes are owls!
Soren's sister, Eglantine, is falling under the spell of a strange nightly dream. Just as Soren notices her trancelike state, the dreams become a deadly waking nightmare that puts the Great Tree of Ga'Hoole in terrible danger.
In the midst of war, Eglantine has unwittingly become a spy for Kludd, leader of the Pure Ones. Brainwashed, she is powerless to prevent Kludd's forces from infiltrating the Ga'Hoole tree - and so a deadly conflict begins...