Bluewing. The Frost Chronicles, #4

Lia Weaver is a fugitive. Her family's farm has been confiscated by Farther soldiers, her sister has been reassigned to a new family in the village, and her official status is "missing." Now, she and a band of fugitive followers must make their home in the harsh wilderness of the Frost. Food is scarce, and hope is scarcer still as Lia tries to find information about the whereabouts of her missing friends.
She is determined to rescue them, but when a surprising ally steps forth with a offer that will both return her friends and expel the Farthers from the Frost in exchange for something very precious, Lia must make a choice.