Note: the plot moves quickly because I am impatient and can't be bothered to write too much buildup, as I also have a short attention span and will give up if it takes too long. This is true of all my stories. Secondary note: Both characters are virgins, despite it never being explicitly stated.
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Every Friday, Jake slept over at his best friend Mark's house. Mark, Jake, and Mark's twin sister Jamie would stay up late playing video games and eating more junk food than their stomachs could bear. This tradition had started when Mark and Jamie got a GameCube for Christmas in fifth grade, and many fights broke out between the boys when they played Mario Party. As time went on, consoles were upgraded, games got more violent, and so did the fights. Jamie, despite being just as good as (if not better than) the boys, was always the peacekeeper. She helped them to keep their heads on straight, reminding them that they were best friends, and if they killed each other, eventually they might regret it.
In their senior year of high school, one thing about this tradition was different. Jamie had always been into gymnastics, and in her junior year, she had made the varsity girl's team. This meant hours of practice after school most days, including Fridays. This left the boys without a mediator - but they somehow managed not to murder each other.
When they were young, Jake had always had a thing for Jamie. Maybe it was because she was the only girl he was consistently interacting with, but this trend hadn't changed with time. At eighteen, she had long, beautiful dark hair, a slim figure from gymnastics, and the most perfect ass he had ever seen. Jake spent more time than he'd like to admit staring at her ass. He tried to never be creepy about it, knowing how terrifying it was for girls to be catcalled or otherwise objectified, but he did take advantage of any moment he got to discreetly admire her body.
Being athletic, she wore a lot of tight yoga pants and loose fitting t-shirts. This fueled Jake's fantasy that she mostly wore thongs - because he could never see a panty line when he watched her walk. However, Jamie wasn't interested in dating, and Jake wasn't interested in getting his ass kicked by Mark if he slept with his sister.
Tonight, Jake and Mark were playing video games when Jamie walked in around eight-thirty.
"Hey guys." She called out, making her way through the living room.
"Hey..." the boys responded automatically.
"I'm going upstairs to take a shower." She said.
"Okay." The boys answered, not even registering what she was saying. With that, she disappeared up the stairs.
Earlier in the evening, Jake had drank three cans of cola consecutively - and was feeling it. He paused the game.
"I need to piss." He declared.
"Okay. I'll make more popcorn." Mark offered.
The boys stood and stretched, then walked away. When Jake reached the bathroom and opened the door, he was greeted by Jamie's bare ass, as she was bending over to take off her socks. They both jumped when the door opened, and Jamie tried to cover herself with her hands.
"What the hell are you doing, Jake?!" She shouted angrily. Jake covered his eyes with his hands and turned around.
"I'm so, so sorry Jamie! I didn't know you were in here!"
"I told you guys I was taking a shower!"
"We weren't listening!" He pleaded. He heard Jamie sigh behind him.
"Just... get out. Don't tell Mark about this, he wouldn't let either one of us ever forget it. I'll be out in twenty minutes."
"Okay. Again, Jamie, I'm really sorry."
"I forgive you. Now, fuck off." She joked, closing the door behind him. Suddenly, he realized that he didn't have to pee anymore.
Jake shuffled back downstairs to the living room and threw a blanket over himself to hide his erection. Mark came back in with another bowl of fresh popcorn and two more sodas. Wordlessly, they resumed their game.
Jake couldn't concentrate. His mind kept wandering back to Jamie standing there, bent over. He had imagined her like that more than a few times - and those few seconds he got to see it for real only worsened his lust. He wanted to see more. He kept wondering wondering what it would be like to have her bent over like that, his hands on her hips, that perfect ass slapping against his pelvis... he tried to physically shake that last errant thought from his mind. Mark would kill him if he knew Jake was fantasizing about his twin sister, especially in such a dirty fantasy.
Twenty minutes of fantasizing later, Jake had gotten soft enough to use the bathroom again. He excused himself and went upstairs, half-hoping that Jamie would still be drying off after her shower. Sadly, she was already in her bedroom.
On his way back downstairs, he nearly crashed into Jamie. Jake took a step back and motioned for her to go down first. She smiled. Jamie had changed into a pair of sweatpants that hung nicely on her frame, a blue sports bra, and she had a towel wrapped around her hair. Jake admired the way her sweats hung over the top of her ass and the swing of her hips as she walked.
Feeling his cock harden, he mentally chastised himself for staring. Once in the living room, she stopped for a moment.
"Mark, where are Mom and Dad?" She asked.