The Republic of Birds / Jessica Miller.

Enhanced descriptions from Syndetics:

A young witch must save her sister from evil birds in this masterful middle-grade fantasy

In the land of Tsaretsvo, civil war has divided the human kingdom from the Republic of Birds. Magic is outlawed, and young witches are sent to a mysterious boarding school, from which no one has returned. Olga and her family live a life of dull privilege in the capital until her father displeases the tyrannical tsarina. The family is sent off into exile at the Imperial Center for Avian Observation, an isolated shack near the Republic of Birds. Unlike the rest of her family, Olga doesn't particularly mind their strange new life. She never fit into aristocratic society as well as her perfect younger sister, Mira. What does worry Olga is her blossoming magical abilities. If anyone found out, they'd send her away. But then Mira is kidnapped by the birds, and Olga has no choice but to enter the forbidden Republic, a dangerous world full of iagas, talking birds, and living dreams. To navigate the Republic and save her sister, she'll need her wits, her cunning--and even her magic.