Yes, he is too young to be bred. At 6 months, he still wil not have developed into the adult dog that you will need to evaluate for stud use. Also, it is ideal for the female to be smaller than the male. Yes, possibly it will change his attitude and an post-mating neuter will not neccessarily correct his behavior as you may hope it would. (Our stud would often go though times of not eating, whining, marking and depression just being around someone who owned an in-heat female.)
Plus, I don't know what the goal of you or your sister is, but generally, wanting to "breed a female just one time and then have her fixed" is not exactly the best reason.
