Growing accustomed to the situation

Sitting there on the couch, having finished for the second time in such a short span of time, you contemplate how it's even possible to still be erect right now. With a small shrug you decide that you may as well make the most of it - whatever magic gave you access to this amazing world of hyper-tolerant women seems to have given you the stamina of a bull. You've blown your load in your step-mother's asshole and coated your half-sister's feet in your seed - at this point there's only one girl in the house you haven't touched.

You find Rose in her room, still naked, counting quietly under her breath as she does push-ups. You notice that her breasts never seem to quite leave the floor, no matter how far up she goes, merely squishing into the floor when she lowers herself down. You also manage to notice a thin layer of sweat across her body, a sheen that makes her skin glisten as though she had been oiled up. Her muscles seem to slide like liquid under her skin as she lifts herself off the floor, before allowing herself to slowly dip down once again as he heaving bosom squishes out underneath her.

"Having fun there?" Rose breaks you out of your staring as she hops to her feet. She gives you a smile and allows you another moment for your eyes to run over her body. Her muscles give her body a beautiful bit of definition that almost distracts from her huge tits - almost. Between her legs, a small patch of hair hides her more female parts from immediate view, causing your erection to throb as though it were being physically pulled towards your sister's holes.

"I guess you're here to **** me?" Completely casual about the whole deal, she beckons you a little bit closer. "Rocking a hard-on like that," she chuckles, "It's no wonder you need it. Hey, I was just about to do some squats - how about you hop under me and we'll see what happens?"

Just like that, from the mere fact that you were standing there with your dick out, your very own sister has already suggested how you might use her body for your own sexual gratification.

"Aren't you a little weirded out?" You start, despite yourself. "I mean, I'm your brother, but you're more than happy to have sex with me?"

Rose makes a face. "Uh, of course it's weird. I don't just fuck whoever, you know. But you're my little bro, so I've gotta look out for you." She gives you a hot, sweaty hug for a moment before releasing you. "Even if looking out for you means letting you stick your cock in me. I mean, a boner like that? After you already blew a load in Masumi's ass and did goodness knows what to Mei? It just can't be healthy for you!"

Before you realise it, Rose is gently pushing you to the ground, setting you up on a soft mat sitting on her floor. Without a moment's hesitation, she is already bringing herself down to a low squat, her powerful thighs resting either side of you as she lines your cock up with her pussy.

"Hey, just tell me when you're gonna cum though, okay? I don't wanna get pregnant - can't imagine how much bigger these could even get," she fondles her breasts for emphasis, lifting them before letting them drop back down again. "Though, I guess it can't be helped if you do cum inside. Anyway ..."

You feel the tip of your cock pressing at her entrance at first, hot and a little slick with sweat, before your sister drops herself down, taking your entire shaft in one go. She begins to bounce, slowly at first, shuffling her legs to get a good position before picking up the pace, her hands resting on her knees as her thighs do all the work.

Her insides are tight - far tighter than you expected. You realize that every time she pushes herself back up, her insides tighten as the muscles contract. It creates this amazing sensation of wondrous tightness as she goes up, and a smooth, slick feeling as she slides back down onto your shaft. Rose is quietly counting under her breath, a number for every stroke - having sex with you has simply become part of her exercise.

You reach up and grab at her breasts. Those breasts you've been fantasizing about ever since you were old enough to fantasize are finally in your grasp, squishy and perfect under your grip, nipples hard under your dutiful care. Looking down at you with beautiful emerald eyes, Rose smiles.

"Hey, that's pretty good for balance. I should have you hold my tits up more often!" she laughs. You see it too now - how much faster and easier she can move on your cock without having to offset the massive weight of her bouncing boobs. With a smile of your own you continue to fondle her, safe in the knowledge that somehow, this is good for the both of you.

"Rose? I'm going to go down to the store, do you need anything?" Masumi appears at the door, having slipped on some clothes to go out. Looking back at her, you realise she hasn't bothered to wear underwear - you can still see her perky nipples through her shirt. She looks down at you with a kind smile, noticing you.
"Oh, John, I didn't see you! It's good to see you two playing together after so long."

Rose chuckles, her voice slightly breathless as she continues her savage **** on your length.

"If John insists on playing with us so often, we're going to need birth control before he knocks us all up! Might be a bit late for me though," Rose smirks. Masumi chuckles along, a surprisingly delicate sound. You feel like Rose might be right about it being too late for her - you can feel your seed building inside you, **** to paint Rose's insides white.

"Masumi," you call, "Give me a kiss before you go." She hesitates for a moment before deciding that it just can't be helped. Kneeling by your side, she leans over and goes to give you a gentle kiss on the cheek, nearly falling as you grab her head and bring her in for a deep and passionate kiss. Masumi goes along with it, her tongue entwining with yours. You can't hold it in any longer, your grip tight on Rose's breast as you begin to cum. Giving a soft moan, you let your step-mother go and place both your hands on Rose's hips, forcing your cock in as deep as it can go as it shoots wave after wave of cum directly into your big sister's womb.

"Hey, I thought you were gonna tell me when you were about to cum," Rose complains, making no move to pull away even as her pussy is filled to the brim with your seed. "I guess it can't be helped. You just had to cum inside me." She gently grinds against your cock, milking out the last droplets of your cum even as she pants from the exertion of her exercise. Finally both of you are still, your orgasm complete. Rose leans forward a little now, taking some of the weight off of her legs and onto her hands, still panting heavily as her breasts now come to rest against your chest.

"I should ... get you to ... help me exercise ... more often ... !" Rose pants. She lingers for a moment as you enjoy the afterglow, catching her breath before she slides herself off of you. Cum drips slightly from her pussy as she stands up and steps away as though it were nothing. She reaches into a tissue box only to find it empty and, with a sigh, turns to Masumi just before she leaves.

"Can you get some tissues? I'm gonna need them if this becomes a regular thing." She sits down on her bed with a thump, bouncing slightly. With no tissues to clean herself off, it looks like Rose is more than content to just leave your seed dripping out of her. After all, it can't be helped.

"Bro ... " Rose looks over at you, "How do you still have an erection?"