"I'll never do it again"? No, no...That's not the right thing to say. Surely what you mean is, who needs the other sports clubs when you can satisfy your newfound harem yourself, yes?
Also, what a great wingman his sister is!
IRL ... "little brother" gets beaten up, captured in embarassing photograph, reported as an extreme pervert, might get suspended or expelled from school, social record downgraded for life, etc.
In H-Universe, ... "little brother" becomes a "Lucky Bastard", who has to satisfy a locker room full of me-so-horny schoolgirls, even at the risk of Death By Snu-Snu.