Based on Season 1, Episode 6 "Skin"
Prompt: "I swear the more I learn about you and your family – I thought I came from a bad background. He's sure got issues with you" – Shape-shifter
The shifter was parading Dean's body before Sam, alternating between hitting his 'brother' and running his fingertip across the man's skin. Cold sweat pooled on Sam's face.
"I know his every thought, his every feeling. I know what courses through his veins when he looks at you," 'Dean' tipped Sam's head up, "But then again, I am him, just without all that morality crap. Which means I can do this-" he paused to run his tongue over Sam's lips, "And not feel bad about it."
Sam spat at the shifter. "You really are twisted". His hands fiddled with his binds, trying to break free.
"No, you see, Dean is twisted. This is what he wants. He wants to covert you, to kiss you raw, to make love to you in the morning light of whatever stingy motel you've landed yourself in."
"Shut up," Sam raged, "Don't put that crap in his mouth – I can see right through you"
"Like you did when you were fourteen, came back into the room dripping in that green blood. Vulcan we joked. Down in Tennessee, remember? And I tried to clean you up, but your sweat was on my hands, and your skin was beneath my fingertips and I ran off into the bathroom to hurl my guts out."
Sam twisted between the shifter's hands.
"But do you know what else Sammy? When I closed that door on you – I didn't want you to see me. Jerking off to thoughts of you; sick with the want of you"
Sam felt the bile rise in his mouth.
"You don't know me at all," 'Dean' spat at Sam, "You thought you were so pure, so innocent – so rid of your big brother and the monsters you fight when you ran away. College." He ran a finger down the side of Sam's face.
"But you never left, you were locked in here, in my dreams – the haunting I could never salt and burn." He pressed his lips forward and captured Sam's mouth in a fierce kiss, before standing back up and punching the man so hard Sam's lip bled.
"Dean really is sick Sammy. You both are," and swiftly the shifter turned and walked off, leaving Sam to his thoughts.
Back at the Impala, after the whole ordeal was over, Sam swung himself into the passenger seat and looked across at Dean.
"You know that shifter tried some real crap to get a rise out of me"
"Like what?" Dean asked, starting the engine.
"Incest"
Dean froze for a second too long then turned to face his little brother, "He what?!"
"Said you wanted to sleep with me!" Sam laughed, "Guess he doesn't know us as well as he thought"
Dean turned away and pulled the car out into the road. "Guess not" he said.
But all Sam noticed was the way his brother avoided his eyes. The white knuckles on the steering wheel. The sweat on his brow.
And he leant back, and said nothing.