The 14 year old boy NAME_1 won a bet against his cute 13 year old little sister NAME_2, who now has to submit to his every whim. NAME_2's boobs started developing last year, were yet small. Her butt was still a small girl's butt, a tuft of hair crowned her otherwise smooth pussy.
"I've got an idea for your school outfit", NAME_1 says to NAME_2. "You can wear my big hoodie like a dress but be completely naked underneath!", he says excitedly. She agrees and they get on the bus to school with the other kids. On the way from the bus to class NAME_1's hand rests on NAME_2's small butt, pulling the back of the hoodie up a bit to flash her naked butt to his friends when they were looking.
Flesh out and embelish the story. Start with NAME_2 getting off the bus.