Ruby gives in, You slide in.

Ruby continues to tease you, testing your willpower to its breaking point with each and every 'accidental' movement where your Sister would take more of your thick throbbing cock into her hot wet sex. Just having your Big Sister's sweet lips wrapping around your swollen tip is bad enough, but when she sinks lower than she should and you feel the hot wet velvet-like walls of her pussy squeezing your cockhead, it's maddening. You strain and struggle, **** to feel more of your Sister, to fill her hungry pussy with your thick shaft, but wanting to prove to both Ruby and yourself that you can control yourself. You can master the seemingly insatiable primal hunger, all too eager to add yet another twisting branch to your family tree.

You look up into your Sister's bright blue eyes, your face contorting with the immense pleasure of your illicit connection with her, your own eyes almost a mirror of hers. Only where Ruby's are twinkling with a devilish glee with what she's doing with you, to you. Your eyes are weary from the effort of holding back, from resisting that urge to thrust upwards and wipe the domineering grin off your Big Sister's face.

"Not having any trouble are you?" Ruby asks with a sensual tone, followed shortly by a throaty moan as she has one of her 'accidental' slips.

You want to plead with your Sister that she needs to either stop completely or at the very least she needs to stop pushing you by taking more than just the tip of your cock into her slick sex. The longer you try to hold out, the weaker your grip on control is getting. Even now you can feel your arms trembling as you continue to claw at the increasingly wet sheets beneath you. The urge to simply reach up and take Ruby here and now, growing harder and harder to push away and ignore. You feel muscles tensing in your body preparing for a sudden burst of movement as your willpower erodes with each sweet forbidden dip you take in your Older Sister's pussy.

Fortunately, before Ruby's teasing can pull you down over the edge and make you give in to those primal urges to rail and ravage your Older Sister, Ruby herself loses control.

"Fuck, I'm sorry Amy, I was supposed to be the one teach you self-control," Ruby says softly between heavy moans.

"Ruby?" You ask hesitantly, while still making a heroic effort not to give in to the deep-seated desires to drive your hips upward and bury yourself in between your Big Sister's sweet slippery folds.

Your Older Sister doesn't even attempt to reply, she simply lowers herself down onto your thick throbbing cock as before, only where she normally kept herself from taking more than the first inch of you, she simply continues to sink down on your shaft. You feel your Sister's hot wet folds parting around your throbbing member, eagerly swallowing inch after dizzying inch of you. Both you and Ruby let out a long groan of pleasure, the pair of you overwhelmed by the sensations of your bodies connecting in such an intimate yet forbidden way.

Soon you find Ruby's rear resting against you, your swollen shaft swallowed up by your sister's slippery slit, the pair of you breathing heavily. "Fuck you feel so big inside me," Ruby pants, her legs trembling from the excessive amounts of pleasure.

"You feel incredible," You pant back, trying to get a grip on your self-control, your throbbing cock, twitching and leaking fresh precum into your Older Sister's deepest depths. "But what about the test?" You ask, uncertainly, feeling a pang of guilt that you made no effort to stop Ruby from impaling herself on your cock.

"Fuck the test," Ruby replies. "Big Sis needed dick too much," Ruby adds with a giggle, the sensation sending a vibration through your cock. "Besides you could have pulled me down onto this magnificent beast much sooner," Ruby reasons, slowly grinding her body in a circular motion and letting out a low hiss of pleasure.

"I guess," You grunt, still not quite sure, but in far too much pleasure to argue.

"We'll figure it out later, right now I think we both need this," Ruby purrs, grinding herself against you, causing your cock to brush around the depth of her sex, your swollen tip brushing past your Sister's cervix.

You let out a groan of agreement, which Ruby takes as a sign to start riding you properly. She places her hands on your chest and begins to buck herself up and down on your shaft, each pass flooding your mind with fresh waves of pleasure. The room quickly filling with a rhythmic wet slapping of skin on skin, accompanied by a sweet melody of your Older Sister's sighs and cries of delight.

You reach up with your hands to grasp your Older Siblings hips, but Ruby slaps your hands away and lets out a sexy giggle. "Uh-uh, I'm in control here," She pants shifting her legs into a more comfortable position as she begins to bounce up and down on you.

Your Big Sister rides you hard for a good long while, before she crashes through her first orgasm, letting out an ecstatic cry of delight as she slams herself down onto your big hard cock. Her sweet folds pulsing and caressing around your length, milking the monster member of any precum you have to offer, while you simply lie there in a delirium of desire and the decadence of taboo pleasure.

"Fuck, it's been so long since I came while fucking," Ruby pants, while still bouncing and recovering from her orgasm. "Although I've never had a cock, quite like this before," Ruby adds with a wicked cackle. "Something about it being your cock, Amy," Ruby begins with a grin. "My own Sister, and not a condom in sight," Ruby concludes. "Nothing between those big swollen balls slapping against my ass, and my poor empty womb," Ruby says in a sensual tone that carries a dangerous edge to it.

"Fuck," You pant, trying to fight your way through the lust haze to reply. "We really shouldn't" You warn, feeling that pressure and heat within starting to build.

"I know, I know," Ruby replies, "But something about it, got it gets me going," Ruby adds biting down on her bottom lip as she picks up the pace of her bouncing once more. "My own Sister, marking me as hers by spraying her hot thick cum all up in me," Ruby purrs.

"Ruby," You groan, hoping that your Big Sister was just fantasising here since you doubt you'll have the strength to push her off you if she isn't.

"Fuck the idea of it being your sperm that knocks me up, Amy," Ruby hisses in delight. "It's almost enough to make me glad my loser ex-fiance wasn't man enough to do it himself," Ruby giggles.

By this point, you can feel the bio-mechanical wheels in your core begging to ramp up in speed, your Older Sister's constant talk about you filling her womb with your cum, having a profound effect on your ability to hold back your impending climax. Your cock twitches deep inside of Ruby's pussy, launching with it a fresh batch of precum against the entrance to her womb.

"You love the idea too, don't you Sis?" Ruby whispers leaning her head down towards yours, moving her hands onto your arms to keep leverage for her still bucking hips. "Our dirty little incestuous secret, slowly swelling my stomach, my big heavy tits swelling with milk," Ruby continues to whisper seductively into your ear.

"Please, Ruby, We can't" You whimper back half-heartedly.

"Oh, but we can," Ruby replies, leaning down and pressing her lips to your soft sensitive neck and lightly sucking on the skin, leaving a small red mark. "I can feel you, so hard for me, cock ready to fill me, pump me full of that thick cum," Ruby purrs into your ear.

"Please, I can't hold back," You warn, feeling yourself rapidly approaching that point of no return.

"But what If I want it," Ruby ask in a soft tone, "Could you really say no to me, Sis?" Ruby asks with an evil smile.

"Please," You plead.

"Please, Amy, Give me a baby," Ruby pleads in a soft needy tone. "Please breed your Big Sister," Ruby adds.

"Please, Ruby, We really shouldn't," You reply through the last remaining shreds of willpower you have remaining, your swollen cock, twitching dangerously.

"No, you don't want to breed me?" Ruby asks teasingly, to which you shake your head weakly. "Well lucky for you, it's the wrong point in my cycle anyway, but kudos for showing restraint, Little Sis," Ruby giggles softly into your ear. "But I still want to feel that cum inside me," She adds before leaning back up and beginning to put all her efforts into riding you over the point of no return.

The sense of relief upon hearing your Older Sister's words is fleeting, as soon you are crying out in ecstasy as your cock begins to throb and erupt deep inside of Ruby's pussy. Ruby lets out her own loud cry of pleasure, as the first thick rope of your white-hot cum splatters across the back of her pussy setting her own orgasm. The hot wet and increasingly sticky walls of her sex, begin to flex and grasp at your cock, milking it furiously while your seed begins to seem into your Big Sister's womb.

"That's it, Amy, fill your Big Sister up," Ruby purrs while patting her flat stomach, a slightly devious smile on her face as Ruby was only partly truthful earlier. As while it might be the wrong time in your big Sister's cycle to be considered her most fertile, it wasn't exactly the safest point either.