Lie to her. She needs a BIG round belly for being a troublemaker

You stare at her peachy cunt for a while longer while imagining the way her brother must've stretched her little pussy just a day ago when he fucked her senseless. The way her insides wrapped TIGHTLY around his dick as if they're made for each other. And eventually the way he squirted oh-so-many loads of cum right into her cute tummy and neither of them were even aware of the fact that she may end up with an baby in there.

It was a whole lot simpler and easier. And both of them enjoyed it.

Your gaze wanders back up to look her in the eyes, and the panic that filled them. There was a solution that will kinda satisfy everyone, you figure. "Relax," you begin speaking to her soothingly. "Did you really forget it all? It was your plan to pretend being his girlfriend. In fact, you were also the one who seduced him..." you trick her, and judging by the way her eyes opened up wide in shock she did seem to believe every word!

"What? No way! Why would I do that!?" she objected while furiously shaking her head. "That's gross! Why would I wanna have sex with my own brother?" she added.

"Well." You raise a hand up to support your chin, fingers lightly tapping your skin. "I dunno, you were a little tipsy and began talking about how nice it would be to have your first fuck. You were like an entirely different person! But you seemed to be happy about it. Any remaining doubts were gone by the time you started having sex, and like, had an orgasm every minute... seriously, Janny. You two fucked like rabbits! It was crazy... but as long as you had fun, I didn't wanna spoil it for you!" She slowly nodded along, and you get the impression that she does at least remember climaxing like mad.

"But what if I get pregnant?" Her expression changes from one of relief, back to one of anxiety.

"It's fine, it's fine..." you respond soothingly to try and help her ease up. The more casual your tone is, the more likely is she to believe that everything is perfectly okay and safe! Psychology basics 101 right there! "Remember the girl from yesterday? She put some spermicide into you, enough to last for multiple days. Each time your brother squirted his sperm into you it simply died off almost instantly. It's all good. You're safe, gee. But you know what? Maybe I can help you feeling even better about it anyway... get on the ground for a moment," you request, and she hesitates before sitting down. "Nope! Just get on all fours, and then put your head to the ground while keeping your bum high up, I'll see if I can find some more spermicide to put into you."

"What!? But that's such a degrading posi-... fine..." after reluctantly considering her choices, she follows your guidance and gets on all fours and lowers her head until her cheek is pressing flat against the cold bathroom tiles. In the meantime, you get back to the medicine cabinet. You already know there's none of the sperm-killing stuff remaining, but maybe you can find something similar to ease her mind... maybe even something better? You look at the different labels and find lotion, some tooth paste, lube... none of which sounded particularly special. You know, something with that certain oomph!

At least until you find a little tube with exactly the kind of label you were looking for. 'Conception Aid'. You grab the smooth tube and begin reading the description. Conception Aid is a special fluid to aid struggling couples who wish for a baby. Our researches found out that the qualities of a healthy man's pre-ejaculate offers the perfect liquid for sperm to swim and survive in, for up to three months! Unfortunately, a woman's natural lubrication serves the opposite purpose and prevents sperm from living on for more than a few days. Therefore, Conception Aid fixes this issue by PERMANENTLY altering multiple aspects of the woman's physical response to arousal. If Conception Aid is applied correctly, the female user will begin producing a new fluid identical to pre-cum instead of the harmful vaginal secretions. It will however serve the same purpose as natural lube, while simultaneously allowing sperm to live for up to three months inside the female. If you no longer wish to get pregnant, it is STRICTLY necessary to use birth control, since the new longevity of sperm will otherwise almost guarantee conception.

Perfect, then. Her pussy will smell like dick all day, but that's already the case anyway if she keeps doing her brother. So she needs to take his cock just once, and it will be enough to put her at risk for multiple months? Wicked. Her vagina will be even better at storing his sperm than his own testicles! That's almost kind of romantic really. If you manage to put this lube into her lady bits, her brother's swimmers will have all the time they could ever need to explore every corner of her teenage pussy.

Just as you're about to turn around, you notice another paragraph of text. Warning! Use the product sparingly. A common side-effect is a numbing of the genital area. Say what now? That's not good... or is it? She used to be a proper Christian girl, so it didn't make sense that she enjoys sex THAT much... if you accidentally squirt more than just a few drops into her, who knows what might happen? She may even start behaving a little more proper again if she stops being so super sensitive down there. A bit more like yourself! Instead of having sex for her own pleasure, she might just end up doing it for her brother's sake. Awesome!

Since she had her cheek pressed against the ground, she couldn't even see you approach until you knelt down behind her and simply shoved the nose of the tube between her labia. The puffy lips parted slightly to make room for the cold object, but you didn't waste even a second admiring how responsive her pussy is and instead squeeze down on the tube, hard!

"Ahnn!" She gasps instantly as the cool fluid squirts into her. "Relax! I'm doing you a favour, all right?" More and more of the liquid seeps into her and already begins to change parts inside her, encouraging her body to produce the semen-friendly pre-cum that will turn her into the perfect host for her brother's squirmy semen. This is her punishment for climaxing so often! "There there, almost done," you calm her down while emptying every last drop of the stuff into her. Some of it was already leaking through her cervix, good and far out of reach of her fingers even if she'd try to get the stuff back out. When it's all deposited inside her, you simply slip the tube back out of her fleshy opening and use your free hand to firmly squeeze her labia together, creating a snug seal.

"Ouch, what'd you do that for?" she objected with a pout, glancing up at you. There were two reasons! For one, you wanted to make sure that the fluid had enough time to finish its job inside her genitals. Secondly, you noticed shortly after injecting it into her that it reeked of the same pungent smell as sperm. She may not be able to remember that scent, but why take any risks? Especially if her crotch really would permanently adapt that smell... it's not exactly a lady's highest desire to lose the charming, seductive scent of pussy and instead carry the same scent as a boy's used masturbation-tissue.

It was sexy knowing that while Janny assumed you are trying to reduce her chances to get pregnant, you were all but making sure that she won't have even a single line of defense left once her brother squirts his baby gravy into her snatch. With every passing second, the fluid soaked into the thin, fleshy membranes of her vag and changed her to be the perfect breeding mare. When you finally let go of her labia after a few more minutes passed, you are surprised to see that nothing spilled out of her! In fact, as you use two of your digits to spread the lips of her pussy, her insides were glistening but all the liquid had been absorbed! You can't help but to grin from ear to ear. It's too late for her. She cutely wriggles her perky bum cheeks, trying to get your attention.

"Are you done? Is it in there?" She looks back at you with her big watery eyes full of hope.

"Yep. It's all in there. You are good to go!" you announce after which you give her bum a playful smack and get her to stand back up. Her peachy cunt didn't look any different, but the subtle scent of sperm that now escaped her puffy slit was a clear sign that the fluid did its work perfectly! Already now, you could see a faint trickle of a watery, translucent goo leaking out of her pussy and down her thighs. Super cool. Real pre-cum!

"Stop staring, that's embarrassing!" She quickly lowers her hand down to her crotch, blocking the view. Aw well, your job is done. "Huh?" With her hand still in place, she lightly pokes the lips of her pussy. "What the..." she pokes her flesh again before slipping a finger just between the tender folds. "What was in that spermicide? I can't feel anything down there anymore!" Nothing at all? Uhhh... ooops... that wasn't exactly your intention...

"R-relax," you answer with a growing lack of confidence in your tone. "It's just uh, an intended effect of the stuff. You know, numbing the area so you can have sex without pain!" Yeah! That's right!

"Oh... I guess that makes sense..." she replies while prodding her vag a few more times, feeling next to nothing. Who knows, perhaps at least the numbing part is only temporary, if she's lucky. Ah well, looks like you unintentionally turned her pussy into the perfect fleshlight for her dear brother. Now it's just a matter of getting them to hook up again...