Play along with her

"Yeah...that's...right," you grunt between thrusts, slowing down as your orgasm subsides. "Mom's pregnant, so why not my sister as well? You're gonna get knocked up, Julie..."

She jerks upright suddenly, whirling around as your cock slides out of her and begins to drip cum onto the hallway floor. "Holy shit, did you just say what I think you... Oh God." She looks nervous, almost panicked, biting her lip as her eyes dart around. "I'm gonna have a baby..."

"Relax Julie," you say, suddenly feeling out of your element. You reach down to pull up your pants before remembering that you didn't bring them; you settle for awkwardly patting your sister on the shoulder. "Girls don't just get pregnant every time they get fucked...they can't. Right?"

That makes sense, doesn't it? There's no way that pregnancy could work like it did in your old world...every girl would be popping out babies year-round...

"THEY DO IF YOU ORDER THEM TO, MORON!" Julie shrieks, grabbing your arm and shaking you up and down.

Oh. You silently curse Charles; that cocky bastard definitely should have mentioned this. It seems like women's sexual submission to men extends to their body functions as well...you aren't sure how that works, but then again you're not sure how ANY of this works.

"Didn't you pay attention in sex ed?" your sister continues. "I know you spent most of your time fucking the teacher, but you should have paid attention to that part at least! Oh God, I'm pregnant, I'm pregnant..."

"Julie! It's okay!" you blurt out, then scramble for something to say next. "Don't freak out, we can handle this, we-" you manage before she interrupts.

"Damn right it's okay!" She grins widely, her new breasts jiggling slightly as she bounces up and down. "I'm definitely freaking out, but that's because I can't BELIEVE you finally knocked me uuup! Oh God, I can't wait to tell Nessa, and Carol, and... Oh, wow."

Well, that was something else. Relieved, you collect yourself and try to get back into dominance-mode.

"Okay, that's enough. Remember the club? Go get changed, and don't forget to tell Mom. We'll leave in half an hour."

She nods, gives you a peck on the cheek, then bends over at the waist. Before you have a chance to react she pulls your cock into her mouth, sucking off the few traces of cum that didn't make it inside her. After a few strokes she lets your sensitive member slide from her lips and goes on her merry way, practically skipping down the hall.

Shaking your head and smiling like an idiot, you head towards your room. After all, you've got to figure out an outfit for yourself...and maybe while you do that, you can start processing his whole 'knocking up girls on command' thing. It might just have an influence on your plans tonight...