Disclaimer: Okay, so sick of writing this. I don't own this, obviously. If I did own it, I would be writing this stuff in scripts instead of posting it on the internet.
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At breakfast the next morning, there was tension in the room. Everyone else just chalked it up to the usual arguments between the two of them, which was basically what it was. The thing that no one else realized was that "the usual arguments" also included sex.
At school, Derek watched Casey flirt with guys, just as Casey watched Derek flirt with girls. When Derek saw Casey go into the boiler room with one of his teammates, he flinched a little, until Emily walked by.
"Hey Emily, how are you?"
"Oh, hi Derek! I'm fine, how are you?"
"I'm alright. What are you doing tonight?"
"Nothing. I mean, not because I don't have anything to do or anything like that, it's just that Casey has a date tonight, so, umm, I don't have anything to do, I guess. Wow, that sounded dumb, huh?"
"No, not at all." So did. God, she's lame. "Do you want to be doing something tonight? With me? Cause the house is going to be empty, for a number of various reasons, if you want to come over and hang out for a while."
"Yes! I mean, that'd be cool. You know, whatever. I could swing by. What time?"
"Eight."
"Okay, see you then."
Derek saw Casey coming out of the boiler room just then, wiping the corners of her mouth. He leaned in and kissed Emily, slipping in a little tongue at the last second.
Casey walked up to the two of them. "Ew, Emily, you don't know where he's been. Come on, I think I have some mouthwash in my locker."
"Nice to see you too, Casey. Probably wouldn't hurt for you to use a little of that mouthwash either, and I think you missed a spot, right here." He rubbed a spot on his lower lip, and then smirked at her. "See you later Emily."
"What does that mean, Em?" Casey demanded.
"He, uh, asked me to come over tonight. While everyone was out." She replied sheepishly, picking imaginary lint off of her shirt. Casey just glared at Derek's back, and took Emily to the bathroom to rinse, since she had been planning on doing it anyways, after the encounter with what's-his-bucket in the boiler room.
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"Derek, dude, your stepsister gives the world's best blowjobs."
"Sean, dickface, do you really think that is something that I would want to hear about my stepsister?" Derek replied, agitated.
"Sorry, but it's true. You could probably ask any of the guys in here, except for you."
Oh, believe me, I know better than any of you. "Is there any guy in here, besides me, who hasn't gotten some sort of sexual favor from my stepsister Casey?" All the guys looked around at each other, waiting to see if any of them would raise their hands. No one did. "Great, that's just great." Derek slammed his locker closed and went out to practice.