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.
"Do you have any tissues up there?"
"Sure, honey," mom responded, handing Marissa some napkins from the glove compartment. Mom wasn't paying much attention as she listened to whatever dad was saying.
David stared at Marissa in disbelief as she pretended to wipe her nose. Then, she passed the wad of napkins backwards to David and whispered, "Finish off back there. Quickly, okay? So I can sit the fuck back down."
David basically froze. "Wha... what?" He didn't move. The napkins fell loosely out of his open hand.
"Oh my god," she said, "You are dumb as a rock." She leaned forward to pick up the tissues, but as her ass lifted, she reached behind her and slipped her hand down David's shorts. She grabbed hold of his shaft and began to sloppily jerk David off. His cock was at full attention now, almost seven inches of dick that the flimsy shorts were powerless to contain. The waistband slipped back as Marissa clumsily rubbed, revealing its pink head to the open air. David's eyes were wide with terror, but he did not stop his sister. He kept his eyes glued forward, freaking out internally and worrying that his mother might turn her head and see them at it. A bead of precum oozed out of the tip of his cock, and Marissa pulled her hand back quickly when it touched her. She held up her hand and flexed her fingers with a look of confusion on her face.
"Already? Jesus, why didn't you..." Then she understood. A strand of precum glistened on her palm. She wiped it away on David's leg and thought for a moment. Then, to David's utter surprise, she held her palm up to her lips and licked. David's cock pulsed in his pants as it lay against his stomach, trapped by the waistband of his shorts.
"I don't want that on my hand," Marissa said, and David was actually disappointed for a second. She lifted her ass, however, and pulled her athletic shorts up tight. Her shorts rode up her ass crack and David could make out the impression of her panties. Marissa's ass became a plumb suspended by her shorts, which she suddenly rammed backward into Sam's lap. She lifted her ass and thrusted again. And again. And again, each time pushing her crotch up the length of David's penis before pulling back and coming down again.
"I'm gonna.. I'm gonna.." David stammered.
"Get the tissues ready you idiot!" Marissa hissed.
David grabbed Marissa's hips and stopped her mid-thrust. His cock, on the verge of cumming, pulsed and released a trickle of precum. "I don't want mom to see," he hissed back. It took all his willpower not to bring her ass down on his cock right then and release.
Marissa looked annoyed. Then, she reached down between her legs and pulled aside the elastic on her athletic shorts. The piece of fabric covering her pussy was darkened by a wet spot. She tried to pull aside her panties too, but her shorts wouldn't stretch far enough. She swore under her breath while David's wide eyes were fixed on an exposed lip of Marissa's pussy. Without realizing, his hand had drifted back to his crotch and he was rubbing his dick through his shorts. Sweat rolled down his cheeks.
What Marissa did next was unthinkable. Before David could say anything, she rolled her athletic shorts down her ass and wiggled them to her ankles. She pulled her panties to the side, revealing a pair of wet, engorged lips, and grabbed David's cock out of his shorts with her other hand. She pressed the head of his cock against her pussy and leaned back, lowering herself completely onto David's lap. His upturned penis was more than halfway immersed in Marissa's pussy. She quickly grabbed a towel and laid it across her thighs to block the view.
"You guys comfortable back there?" Mom asked, taking a glance at them through the mirror.
"We're fine, I guess," Marissa responded.
"Don't crush your brother! We'll be home in less than thirty I bet," and then mom went back to talking to dad. She didn't notice Marissa's read cheeks or David's glassy expression.
Marissa tried to move up and down on David's cock, but she couldn't fuck him very well while leaning against his chest. Instead, she let the car do the work. Each bump, each vibration, brought David closer to cumming. Meanwhile, Marissa slipped her hand into her panties and began to play with her pussy. She rubbed along the front of her pussy a few times before circling her clit discreetly with her fingers. David thought he would go crazy. For minute after minute, he sat on the edge of cumming while Marissa absentmindedly played with herself and checked her phone.
Finally, David had had enough. He grabbed his sister's hip and thrusted her downward. He stuffed the full length of his cock into her pussy and aggressively fucked her. Her eyes clamped shut and her mouth hung open as she breathed rapidly but silently. It only took about ten thrusts. He went rigid like a diamond and came hard into her pussy. He gripped her hips so hard that he left a mark on her, and she came shortly after, shuddering slightly on top of him and working her arm into overdrive.
When both were spent, Marissa raised her hips and slipped David's cock out of her. She wiped it clean with the wad of napkins and tucked it back in his pants. Her panties soaked up the brunt of the seeping cum, but she wiped just in case. Then, after a few moments so as not to make too much noise, she rolled her athletic shorts back up. David got one last look at her round ass before she leaned back again, panting lightly and taking another look at her phone. David merely closed his eyes in satisfaction.
The two eventually fell asleep. Their parents eventually noticed their kids huddled against one another in the back and sighed with pleasure.
"Oh, it's just like when they were kids! Aren't they sweet? I'm so glad they get along.
"They'll have each other's backs for life I bet, not all siblings are like that."
"Get a picture, honey. We'll probably never see something like this again. They'll be so embarrassed when they wake up!"