Orphan queen duology

When the Indigo Kingdom conquered her homeland, Wilhelmina and other orphaned children of nobility were taken to Skyvale, the Indigo Kingdom's capital. Ten years later, they are the Ospreys, experts at stealth and theft. With them, Wilhelmina means to take back her throne. She is a spy. Wil and her best friend, Melanie, infiltrate Skyvale Palace to study their foes. They assume the identities of nobles from a wraith-fallen kingdom, but enemies fill...