This was originally a Wincestiel fic, but I have rewritten it to be pure Wincest.
Sitting on the bed, Dean trailed his index finger down Sam's stomach, traced the head of Sam's cock, and brought it to his mouth, sucking on the droplet of precome. "Damn, you taste good."
"Get on with it." Sam's grin was pure challenge.
"Oh, I intend to." Dean went to town on Sam. He flipped Sam on his hands and knees and rimmed him until Sam was clawing the sheets and grunting, "Fuck. Fuck. Fuck." into the mattress, until Sam was hard again. He worked himself underneath Sam and took his cock in his mouth, gripping Sam's hips and making him fuck his mouth, moaning all the while. He sucked Sam's balls into his mouth one at a time, pumping the head of Sam's cock in his fist and thrust three fingers into his ass at once, working him slow and deep.
Sam fucked himself down on Dean's fingers, his hands roaming all over his body, as if seeking something to do, something to grab onto. "Dean. Wanna come."
Dean slowed his movements. "Not yet, Sammy. Not 'til I say."
Sam moaned. "Dean. C'mon."
Dean pushed Sam over onto his back and crawled up, straddling him. "What'd I just tell you?"
Sam's breath was rapid, and his pupils were blown wide. "Not 'til you say."
"'S right." Dean held himself up so no part of him was touching Sam's cock, took Sam's face in both hands and kissed him, tracing the seam of his lips with his tongue, making Sam open to him.
"Gonna make you come so hard for me, baby boy. Promise. But not. Just. Yet." Dean kissed the hollow of Sam's throat. Sam arched into it. "So fucking beautiful." Dean trailed the tip of his tongue lightly along the muscles of Sam's arm, lifting it, turning it, moving so he tasted every inch of skin. When his tongue danced along the crook of his elbow, Sam exhaled through clenched teeth.
Dean teased the sensitive patch of skin, licking gently and blowing his breath across it. Sam thrashed and cried out, grabbing a big handful of blanket and pulling hard.
Dean just grinned, mouth still sealed on Sam's arm, tonguing the sweet spot.
"Dean. Fuck. Let me come. Please."
Dean bit down gently. "Not yet."
Dean worked his way down to Sam's wrist, licked between each of Sam's fingers. Sam's voice grew hoarse, crying out in surprise and pleasure.
Dean kitten-licked down Sam's chest, and rolled him onto his side. He licked lightly all up and down Sam's side, lips brushing his skin the whole time. Sam panted and shook.
When Dean moved Sam's upper arm out of the way and moved higher, Sam started to protest. "Dude, what the hell–" But Dean's mouth went where Dean intended it to go.
"Fuck!" Sam writhed, hips rising off the bed.
Dean licked and kissed and nipped at Sam, showing no mercy. Sam bucked and writhed and bit the pillow.
Sam's cries were closer to screams now, guttural, desperate, racked with pleasure so acute it bordered on pain. "Dean. Gonna die."
Dean pulled off and kissed Sam. Sam arched up into him, sucking on Dean's tongue, hands darting frantically along his back, clutching his arms, gripping his ass, thighs spread wide, begging with his entire body.
"So good." Dean bit Sam's lower lip.
Sam moaned into Dean's mouth.
"Wanna make me happy, right, Sammy?"
Sam whispered, "Yes."
"Wanna see you come for me. But first…" –at that, Sam emitted a sob–"First, I gotta do your other side."
Dean moved Sam so he was laying on his right side, and trailed the tip of his tongue along the curve of Sam's shoulder. Sam was now so sensitized that he shivered at the lightest touch.
"Hold still, Sammy."
"Can't. Too much. Feels too fucking good. Christ, Dean…."
"I'm gonna hold you down, Sammy. Help you take it."
Dean used his tongue and mouth on Sam's left arm, twining his tongue in the crook of his arm. Sam spasmed, pulled against Dean's hold, writhed, drenched with sweat. When Dean stretched Sam's arm out, Sam fought back. Hard.
"Can't. Too much."
"Yes, you can."
Sam rubbed his head against Dean's chest like a cat trying to get his attention. "I can't, Dean, I can't, I can't…"
Dean let go, settled alongside Sam and held him, stroking his hair, kissing him slow and sweet. "Shhh… s'ok, Sammy. Love you so much. So good. Drive me crazy. Love seeing you fall apart for me." Sam shivered. "You like that? Losing it for me?" Sam nodded. Dean brushed the hair out of Sam's face, and rubbed his thumb over Sam's mouth. "Can you take a little more for me, Sammy?"
Sam blinked his big brown eyes, transfixed by Dean's hunter-green eyes. "Yes."
"Ok, Sam, you can fight being held down as much as you need to, make as much noise as you want. But I'm gonna do this. And you're gonna take it for me. Yeah?"
Sam sucked in a deep breath. "Yeah." Once more, Dean kissed Sam (because other than torturing Sam with pleasure, kissing Sam was Dean's favorite thing in the world). "Ok, baby boy, here we go."
It was a good thing the motel they were in was entirely vacant other than themselves. The first touch of Dean's tongue on the sensitive flesh underneath Sam's arm sent him howling. Dean scraped his teeth along the skin and Sam convulsed. Dean lapped at Sam over and over, nipped at him, swiping his tongue in broad strokes, and Sam sobbed with pleasure and need.
Dean pulled his mouth away and stroked Sam's side. "So good, Sammy."
"Dean." Sam's voice was wrecked. "Please."
"Ok, Sam." Dean took the bottle of lube from the nightstand and squirted some directly on Sam's cock, desperate for attention, smearing it over him with a light touch. He liberally slicked up the fingers of his right hand. Sam shivered at the sight, knowing what was coming.
"You've been so good, Sam. So good. You earned something special." Sam went to roll onto his back, but Dean stopped him. He slid three fingers inside Sam's ass. Sam was tight, but took all three without trouble. Dean slowly worked in his pinky as well, four fingers, just to the first knuckle, and gripped Sam's cock with his other hand. Sam couldn't even make a sound, could barely breathe, his whole body quivering like a guitar string.
Dean leaned over, bracing his chest against Sam, and licked at Sam's sensitive armpit again. He drove his fingers deeper inside Sam, reveling in the feel of Sam's ass stretched tight over his fingers, his fist sliding over the head of Sam's cock.
"Come for me, Sam." He sealed his mouth against that oh-so-sensitive patch of skin and sucked and licked simultaneously. Sam sucked in a deep, shuddering breath, and screamed. And came. And came. And came.
He came so hard, it hit the wall behind him.
He came so hard, body convulsing like it had been electrocuted, that Dean came, untouched, at the sight.
He came so hard, clenching Dean's fingers and thrashing, he sprained Dean's wrist.
He came so hard, he lost consciousness for four seconds.
When he awoke, Dean was wrapped around him, kissing him, stroking his hair, murmuring words of love and praise. When he saw Sam open his eyes, Dean pressed his forehead against Sam's and exhaled. "You ok, Sammy?"
They lay in silence for a long time, listening to each other's breathing. Finally Dean stirred. "So. You really think you can make me come harder than that?"
"Damn right I can," Sam grinned sleepily. "Show you."
"Like to see you try," Dean murmured.
"It's a bet," Sam managed to say before sleep overtook them both.