The Collar and the Cavvarach (Krillonian Chronicles Book 1) by Annie Douglass Lima

Bensin, a teenage slave and martial artist, is desperate to see his little sister freed. But only victory in the Krillonian Empire's most prestigious tournament will allow him to secretly arrange for Ellie's escape. Dangerous people are closing in on her, however, and Bensin is running out of time. With his one hope fading quickly away, how can Bensin save Ellie from a life of slavery and abuse?