The Searcher — F.M. Parker

Sam Tollin is only sixteen when Comanches kill his parents, leaving him and his thirteen-year-old sister utterly alone in the lawless lands of the Southwest. The two make for easy pickings, and before long they're been kidnapped and bound for the slave market in Mexico City. Through a blind stroke of luck, Sam escapes, but he's forced to leave his sister behind. And he knows it's going to take more than luck to get Sarah back. Bent on vengeance, Sam joins a ruthless band of outlaws who promise to help him with his quest...if he can survive.