There is an interesting plotline that the brother still doesn't really see themselves as a girl, he was always in love with the brother and internally, he's still in love with him and sees himself as a dude. Even mom just treats him as she always did, but now he needs bras. But the Big Brother... I wonder how much he is thinking with his dick.
If the little bro magically turned back into a boy in the finale not much would change for him, he'd still love his Big Brother with all his heart. But I'm pretty sure the Big Brother would finally come back to his senses. The little brother is doing everything for love, even changing their body, but is that a good thing in the end?