Lyra wins big and spends the night giving herself the best chance to knock you up

You spend an enjoyable day teasing Lyra while walking around and seeing a few attractions in Vegas, a brief trip around the wax works, a few hours soaking up the sun by the hotel pool, and some good food. In the evening you take in the glittery spectacle of all the neon delights of the main strip before finally retiring to your room to enjoy an evening of gambling with your Sister.

There's a little bit of waiting around as Lyra sets up some recording equipment, thinking quite reasonably that the fan site, both normal and taboo would eat this video up. You're a little piece to the camera explaining the rules of your game, making the perfect trailer. To the fan site, you made up a story about Lyra sweeping you off your feet these last few days, and you giving her a chance to breed you was a fun reward and a good way to get back at your fictional husband's inattentiveness. To the taboo site, they got the behind-the-scenes discussion of you and Lyra making up a story since the truth couldn't be used.

Needless to say, once everything was done and set up, Lyra could hardly wait, pouncing on you the second she could, climbing between your legs and devouring your pussy. You lie back howling and groaning in delight as you enjoy your younger Sister's expert tongue flicking all over your juicy sex, her sweet lips wrapping around your swollen clit and sucking and kissing both it and your outer lips. It's not long before you feel that welcome heat and pressure of what will be the first of many orgasms of the night. However, just as you feel yourself nearing that point, Lyra pulls back and shuffles into position, much to your protest.

"Sorry Sis, It won't count unless I'm inside of you first," Lyra giggles as she swiftly aligns her cock with your slick slit and slides herself inside you.

Any protest you have about being so close to cumming, only to be cruelly denied by your Sister evaporates as the sweet sensation of your Sister's mammoth member filling your hungry hole is more than enough to eke out the first orgasm of the night. Your fingers dig into the soft skin of your Sister's back as you begin to cum hard on her cock, Lyra giving a soft chuckle and a few kisses while she waits for your pussy to relinquish its iron grip on her solid shaft so she can start fucking you.

"One down, just 8 more to go to give my sperm the best chance at knocking your ass up," Lyra says in a sensual tone.

"Fuck I still can't believe how good you feel," Lyra groan while grinding herself in small circles deep inside of you.

"Might be the first time I'm happy things played out the way they have," You reply honestly, briefly thinking back to how miserable your life was just a few weeks back.

"We'll see if you still feel that way after I've knocked you up," Lyra growls.

"Well this might be your only chance, so better make the most of it," You tease back, causing Lyra to pick up her pace as your words have the desired effect of her.

At first, you didn't worry too much about the game, since 9 orgasms felt like a substantial amount of time to make you cum while your Sister's cock is inside you enduring your sweet walls wrapping tightly around her trying to milk her cock. Sure Lyra has a lot of stamina and can cum quite a few times back to back with little to no downtime between. So while you lie back enjoying yourself as your Sister fucks you to orgasm after orgasm, occasionally stopping for a few seconds to roll you into a new position, where her cock seems to reach even deeper inside of you, you don't worry about the bet.

You're carefree as she sucks your sensitive nipples, while you rock back and forth in her lap in the cowgirl positions. You're carefree as she takes you on all fours while leaving big red hand marks on your soft ass as she spanks you while fucking you hard. You're still carefree as Lyra fucks you to and through your 6th orgasm and showing no obvious signs of being close to cumming while she ramps up the breeding dirty talk. You're carefree since you're oblivious that this bet while being some mostly harmless fun for you, was playing right into your younger Sister's biggest fantasy. You're carefree because you have no idea how determined Lyra is to breed you, to start a family with the big Sister she's idolised and fantasised about for so long now.

To you, your mind cast adrift lost in the deep sea of lust and pleasure, it feels like you merely blink and your cumming again, then just as your eyes close one more you find yourself howling your Sister's name as loud as you can as her majestic cock has you climaxing yet again, your entire body aches slightly from all the muscles tensing and relaxing from the previous orgasms combined with being rolled into new positions your body almost too weak to move without your Sister's assistance.

"Just one more to go," Lyra groans, her face contorting with concentration as she does her best to hold back her orgasm. The 8 previous orgasms, tightly gripping and caressing her rock-hard cock, each trying desperately to coax your Sister's cum from her big weighty balls colliding heavily against your soft rear.

You're not sure if it's your Sister's words that finally get through to you, or perhaps there's enough of a lull in pleasure clouding your mind as Lyra rolls you onto your back and lifts your legs back before climbing on top of you in a press-up position, a position you now know to be called the matting press. In any case, there's a sudden realisation and panic that grips you, that this sordid taboo fantasy might become a reality. A mixed sensation of equal parts fear and excitement rolls through your body as Lyra lowers herself down and sinks into your soaking sex with ease.

The hit of adrenaline allows you to focus through the immense pleasure as your Sister begins to pound you in such a tight and intimate position roughly. The pleasure is very much still there and far from something you can simply shut out, but you can at least start to notice that your Sister's stamina isn't quite as endless as it seems. Through the smug grin on her face, you see the micro expressions where she's fighting hard to maintain control of her own impending orgasm. You feel Lyra's cock twitch heavily deep inside of you and watch Lyra's focus shift as she has to concentrate hard not to cum.

This all means you at least stand a fair chance of getting out of this game without having to wait the hours-long trip back giving your Sister's sperm more crucial time to possibly need. Or at least that's what you believe until Lyra reaches down with one hand and begins to tweak your sensitive nipple with one hand causing you to let out a loud uncontrolled gasp of pleasure.

"Oh, Does someone have sensitive nipples," Lyra grunts with a soft laugh. "You know I hear that can happen around ovulation," She adds with another laugh.

You let out another groan of pleasure, feeling that familiar heat and pressure rapidly building inside of you, while your mind begins to panic at your Sister's words. Could Lyra be right? Could you have inadvertently put yourself at risk at the most fertile point in your cycle? You desperately try and do some quick mental maths, a task made infinitely harder by your Sister's unrelenting pace.

You try your best, but you can't be certain, with all the stress and feeling down, your last few cycles haven't been quite so reliable. This in combination with the sheer wave of pleasure from Lyra working your sensitive nipples while fucking you so well makes it impossible to be sure of anything. It's impossible for you to untangle if you're horny because of the forbidden nature of everything and added risk of it, or if you're hormones were trying to influence you into fulfilling the biological imperative.

All you know is that, if Lyra is right and you're ovulating her sperm from the broken condom might already be ready and waiting and the hours between now and when you get back tomorrow might be the time Lyra's swimmers need to fertilise you and for the egg to implant before the pill can prevent it. The realisation should make you panic and try and put a stop to this game, yet instead, you feel a fresh wave of excitement as your body begins the tense up for your 9th orgasm of the night.

"Oh fuck, just a little longer," Lyra groans her cock twitching heavily and dangerously deep inside you.

It was over, the final moments revealing just how close you come to not taking a big risk, as you let out a loud cry of ecstasy as you begin to cum hard on your Sister's cock.

"Fuck Yes!" Lyra cries out triumphantly as she presses herself as deep into your quivering and convulsing folds as she can as she unleashes an almost superhuman amount of cum into you. The pair of you moaning and groaning in unison as your bodies work in tandem to try and ensure that your Sister's potent futa cum gets to where it needs to be. You lie back and simply bask in the carnal delight of it all, the sweet feeling of the warmth as your Sister fills you completely.

"Fuck that was amazing, part of me hopes I didn't knock you up so we can do that again," Lyra says between heavy panting as she tries to regain her strength.

"Well even if you have, it's not like you'll want to stop at one," You reply with a half smile.

"How serious are you right now?" Lyra asks with a smile.

"About as serious as you were," You reply with a soft sigh as you snuggle up against your Sister, deciding not to worry about things and let nature decide if Lyra will get her chance to try again or if she'll have to wait a long time.