This was going to be part of a larger arc involving Derek's mother getting re-married, his Epic Retaliatory Downward Spiral, and Casey's attempt to "fix" him (which seemed a more comfortable option than "comforting" him) by way of making him fall in love with Sadia when the self-destruction begins in earnest --which inevitably backfires, catalyzing a Sex Explosion that surprises approximately No One.
Even had a tag lined up for it: "What part of tiny leather pants and fish-nets don't you understand, Case? Of course I slept with her."
But all I've been able to write for it in the...Very Long Time it's been on my hard drive have been the Derek-making-out-with-Casey bits.
Derek knows she's there. His Casey Senses are tingling; the room smells suddenly of cake, and he feels the weight of unseen eyes pressing into him, watching.
Sudden inspiration strikes him; it's typically something he avoids, but he's willing to make all kinds of exceptions for his step-sister.
Sadia's saying something he's been mostly ignoring when he silently interjects, hoisting her up onto the counter and sliding her backward, teeth at her throat before she's fully settled, and he feels that aching-sweet stab of heat when he catches a glimpse of Her reflection in the kitchen window, a fault line of anguish stretched across her face. Sadia makes a low, drugged sound in the back of her throat when he tugs her forward, meeting her violently against the lip of the counter, eyes focused on the image of Casey, realizing she's being watched.
She stands stiff, frozen in terror, and he levels his gaze at her, heavy with impiety, holding her captivated as he cinches Sadia's legs around his waist and wedges into her, eliciting a sharp, startled noise and fingers clenched painfully in his scalp.
When he pries his eyes open again, several seconds later, he's alone in the window, and he smirks as he pulls himself out of Sadia's embrace, abruptly, indifferently.
He leans back against the counter, eyes burning through the empty space where Casey'd been standing seconds prior. Belatedly, he tilts a lazy grin up at Sadia.
"Sorry, what were you saying?"
Definitely still in high school, at home.
(And PDA's the "something" he typically avoids. In case anyone was wondering.)