Read in 6th grade about a Spanish highborn son that gets in trouble with the court (king's court). the family goes to the uncle’s hacienda in the new world, he and his brother are captured in an Indian raid, he ends up with a Comanche family and becomes a member of the tribe, he falls for an Indian girl possessed by a cruel Comanche, he goes on a raid to steal horses to buy her.
I think this is by John and Patricia Beatty (or Beaty). I don’t remember the title, but it had a yellow cover, if that helps!