Dad hefted the last cardboard cube into back seat of the SUV. He wiped his brow and sighed.
"Looks like I underestimated how much crap you have, David."
David, 19, had just finished his first year at college and his mother, father, and Marissa, his sister, were helping him move out of his campus dorm. Marissa was a junior in college who had gotten out of class the week before, and she was not happy to be dragged along on this trip. It was hot for May, and she had on a short t-shirt that stuck to her body like cling wrap and exposed her midriff. Her smooth brown hair was tied in a knot behind her head to keep the sweat at bay, and her round ass was contained by a pair of athletic shorts. They barely covered her thighs, and whenever Marissa bent over to pick up a box, the shorts would ride up her ass and expose her cheeks. A little packet of cloth outlined her pussy between her legs. David tried not to stare.
"You'll have to throw some of your junk out," Marissa said, glaring. She was leaning against a tree for shade, her arms wrapped under her boobs. "Either that or sit on the roof." David's stuff took up most of the car. Only a single seat remained in the back between the stacks of boxes, bags of clothes, and TV.
"Oh, the drive is only an hour," said dad. "Just squeeze into the seat." Marissa groaned, but she didn't argue. They squeezed into the back seat, one next to the other, and began the ride home. Mom and dad talked about some plans while Marissa hunched over her phone. David's phone was dead, so he just sat there squished between cardboard and sister. After a few minutes, Marissa nudged David's arm.
"My leg is going numb, move over."
"I literally have nowhere to go here."
"Fine, I'm sitting on your lap then." Marissa lifted her ass and sat herself down on David's left leg. It was actually more comfortable this way, and David enjoyed the feeling of his sister's firm butt against him. She leaned forward uncomfortably at first, but eventually she decided to lean back and rest against David's chest. She relaxed her legs too, and soon her pussy was radiating heat onto David's leg.
She went back to browsing Instagram, but suddenly her eyes went wide and she stifled a gasp of surprise. David held his breath and tensed his fists, but to no avail. An erection was growing under his shorts. His growing penis pushed up against Marissa's ass cheek. Marissa raised her eyebrows and looked sideways at her brother, but David didn't meet her gaze. He mumbled "sorry" under his breath while his face went beet red. Mom and dad continued talking--they didn't notice a thing.
David was beyond embarrassed, but Marissa did not react further. She thought that if she ignored it, the boner would settle down quickly. She continued to scroll through Instagram halfheartedly, but with every bump in the road, David's situation only grew worse. She tried to lift her ass off of David as best as she could, but it wasn't enough. David thought he was going to cry when Marissa suddenly spoke.
"Mom?"
David thought his heart stopped. He grabbed Marissa's arm tight, but she continued.
