Be direct and tell her to cut it out

You watch her pondering her choices, and you get an increasingly bad feeling about this situation. "Come on sis, this isn't funny!" you point out to her, after all your masculinity was on the line. "I want my huge dick back, and then go back to fucking chicks instead of being the one that GETS fucked."

"You're right, this isn't funny. It's pretty serious. If you still haven't learned some respect for what girls have to go through and how easy it is to make a mistake, then it's time that you learn the hardest part. Time to learn to take responsibility. If girls are careless about spreading their legs, they get knocked up," she says sternly before you feel her fingers pinching down on the rubber dome and squeezing a rapid stream of baby gravy right through your cervix. "No second chances for you, baby sis."

"N-no! Why?! I thought you were on my side!" you protest but it was too late! Shit!

"Of course I am, and now you are part of the team, girl!" She moves up and gives you a sensual kiss on the lips to silence your complains. In the meantime, her hands shift around to grab your pillow, and you feel her sliding it away to push it under your bum instead, elevating your crotch and making it easier for your future siblings to swim inside.

"Maybe next time you'll keep your legs shut," she winks at you, while you feel a shiver running down your spine at the sensation of your dad's slippery spunk oozing inwards. Perhaps it was a part of your female body and instincts, but you felt oddly submissive and shy, and couldn't muster any proper resistance to this treatment.

Gwen lay down by your side and spent the next few minutes cuddling affectionately with you, while also making sure that you kept your legs snugly closed to avoid any spilling. Both of you look at the clock nervously, but for different reasons. Only one of you is happy when the last hour ends, and your sister looks back at you. And your tits. And your inseminated cunt. "Well done you lil' breeder. I'm gonna give dad some congratulations later, he did his job!" Gwen was grinning from ear to ear while saying it, and energetically jumped off the bed to join your parents downstairs while you're left to contemplate the situation.