Big bro getting turned into little sis' bitch over summer break; force-femmed, un-potty trained, and stuck in quadruped bondage.
By the time the parents come home, his mind, much like his cookie-plugged ass, is completely broken, unable to go back to his old life.
At least he eventually gets housebroken again, though back in diapers every winter as it's too cold to let 'her' outside to potty in the backyard.