Slowly

You climb off the bed and kneel beside her, grabbing her thick shaft with one hand while pushing up her t-shirt with the other, exposing nipples as erect as her cock. You start pumping her rod slowly while teasing the tip, allowing the ample amount of precum to coat your hand. At the same time, you were teasing her breasts, alternating between brushing your thumb over her nipple and rolling it between your thumb and forefinger. You watched as Gwen tried to fight to keep the abject pleasure from her face.

"Hnng.. f-fuck... wait..." she moaned

"Why, doesn't it feel good?" you whisper in her ear, with a playful nibble.

"Yesss...." she obediently answers "B-but I don't want to cummm like thiiis~"

"Hmmm... sure." you abruptly let go, leaving her rod twitching impotently in the air.

"H-huh?..."

"You said you didn't want to cum like this"

"B-but I didn't want-..." she trailed off

"So you DO want to cum?" you asked, resuming your teasing

"Hnng.... don't do thaaat~" she moaned

"Don't do what?" You stopped again.

"Stop.... stoppinnng..." she finally lets out

"If you insist...." you start jerking her off again, but now you were varying the speed, slowing down when she got close to cumming, but never quite "stopping". Seeing how helpless Gwen was had unlocked your sadistic streak, and you were soon fantasising about all the stuff you could do to her,

"Hnng... l-let me cum you fucking bitch..!" she protests, rattling her cuffs

"Don't worry big sis, I intend to... " You quickly leave her and head back to your parent's room, opening the "sex box" and pulling out a gag, lubricant and a small wired egg vibrator before heading back to Gwen

"W-what are you do-mmpf!" You **** the gag into her mouth. She might get loud you think, as you flip her onto her front.

She jumped a little at the cold lube on her anus, and you took your time massaging it into her rosebud, before pressing a finger into her surprisingly clean ass. I guess the witch gave her a new asshole too A low groan of pleasure and humiliation leaked from the gag. You pushed into her ass with your middle finger and curled towards her stomach, and heard her breath catch. Massaging that point in her ass elicited sounds of confused pleasure from Gwen. Of course, if she now had a fully functional penis and testes, then she would also have a prostate , made extra sensitive like the rest of your bodies. You took the bullet vibrator and pushed it slowly into her ass until it nestled against where your fingers were. By now Gwen realised what you were doing and you got to enjoy watching her rosebud pulse in and out in a vain to dislodge the toy before you could turn it on. You flipped her back over, showing her the remote in your hand as you turned it on.

The reaction was near immediate. Her hips jerked as her back was arched taut. Her cock practically jolted upright, and was leaking more than ever before. Her tearful eyes were wide with surprise and ecstasy. Normally this much stimulation would've been enough to make the already sensitive Gwen cum immediately, if it weren't for that witches rule that said "no cumming on your own". You correctly guessed that it included being brought to climax remotely by a toy, even if controlled by someone else. Satisfied, you turn the vibrator to low and pull the gag off her.

"Nnnnngh~... what did you dooo~" she moaned

"All the plumbing of a dude..." you reply . You stroke her tummy, feeling the vibrations through her abdomen. "How does it feel?"

"F-fuck.... cumming...." she slurs. You see her spasm, not unlike your own climaxes.

But that's all that happens. No ejaculation, and her cock is still as erect as ever. You turn it to high, making her "cum" again for good measure.

"St-stoppp..." she complains

You turn it to low "But didn't you want to cum?"

"B-but not like that.. please just let me cum, let me cum..." she finally breaks, sobbing quietly.

Damn, now I feel a little bad... you felt a twinge of guilt in your chest seeing your sister reduced to such a pitiful state by your hand, even as you felt a sadistic twinge in your sex. You finally decide to put her out of her misery. Afraid she might wake up the neighbours, you put the gag back in place and shuffle to the foot of the bed. Your head hovers over her shaft, lips touching the tip before suddenly bobbing down, swallowing up to the root in one go. At the same time you turn the vibrator to high. You pull your head back up, almost to the tip, before going back down again. Barely three bobs later you feel her twitch like mad as she finally starts cumming for real. You hear her screech into her gag as your cheeks puff out with her pent-up seed. In greedy gulps you swallow it all, the sensation of it going down being enough to start your own orgasm. Her shaft continues throbbing in your mouth as you resume bobbing up and down. Gwen yelps in surprise Your finger re-enters her ass and pushes up on the bullet vibe as you continue draining her balls. She wanted to cum, now she was going to give up every last drop. You feel the contractions around your finger as her cock throbs again in sync, and you feel a second, much reduced load fill your mouth. After a few minutes of this repeating, you figure she's completely empty and with a sucking 'pop' you release her and turn the vibe off.

"Mmmm... you know how good your cum tastes Gwen? "...Gwen?"

you kneel up to get a look at your sister only to see her eyes closed, with a semi-peaceful expression on her face. Apparently your **** had taken more of a toll on her than you thought, and she'd either passed out or fell asleep near the end.

Hmmm... not sure what to do now...