Honestly I don't like the explanation of the brother just being a genius... At least they could say other actually motives that make more sense he grow up taking beatings of Setsuna every day for years, and also did many different sports. Sure add a little of the genius and being quick learner but his base should already be there just from taking her sister's indirect "training"...
The MC until now was a genius already, it went from being a weak nobody to win every fight he did... And now this guy is like the ultimate genius? He should be stronger than his sister I guess if he cared about fighting...