My favorite is the big-butt brother who raises his little sister to have a big butt - Mari Takasugi MIAE-283

There's nothing more gratifying than my sister's well-developed buttocks. From a young age, I (her older brother) have been giving her an elite education on her huge buttocks, and she wholeheartedly believes in this, doing buttock exercises every day. To help her maintain her balance, I also use my penis to fully guide her.