Nothing that doesn't cum with the territory...

Cassie continues stuffing herself into you at a slow pace. She seems to be enjoying herself immensely as you know she is about to cum again.
"Your butt is too good to hold it," she explains as her cock only went in three inches more where you feel her shoot. You feel concerned again as she still seems to cum as much as the first two times. Where is the safety mechanism? You can't hold this much without bursting! ...Right? You see your belly bulging out, but you don't explode. She pats you there.
"I'm glad you have a high capacity, bro! This makes me feel good to know that you will be able to keep your word to take my cum whenever I need you to!"
"Well it won't be as bad as a two foot long erection would be coming out of your crotch, Cassie."
"Right. You think of everything, it's so nice to be taken care of," Cassie swoons. "Speaking of two feet, I gotta get this thing moving."
She seems to have a wider, lustier smile on her face as she continues burrowing her penis into you. Once again though, after only maybe three inches, she stops to cum, breathing heavily, her small tits wiggling. After recovering, she admires her handiwork again.
"I didn't know I could do this," you tell her honestly.
"Well you're my brother, of course you can! We were made for each other, after all," she tells you as she jams more dick into you. You feel her balls brush your ass as she cums again. She's almost in. You center your heels on her butt cheeks and pull her in, her crotch tightly fitting between your legs and her balls pressed into your ass.
"Ah wow, I never thought anyone would take it all! You are so full already, but I need to cum more still! My pregnant brother, it's like I knocked you up. It's gonna be awkward for you, us disappearing for a few minutes only to have you end up like this!"
"I don't mind as long as it keeps you from popping a huge boner in public. A big belly is not as bad as that."
"Such a good brother! Good brothers get rewarded!"
You feel her balls pulse against you. You recall having a thought that it would turn out this way.
"I like it, actually," you blurt out. She shoots you an odd look.
"You like looking like a girl?" She rubs your huge cum bulge as she asks. "You did get a lot of sexy clothes. We could try sharing them. I've always felt like a freak, but if you are too, then it makes me feel better." She starts fucking your butt now, with long powerful strokes. "You are getting fucked by your sister's giant monster cock. A big pregnant belly seems like it would cum with the territory."
"That's a good idea, Cassie. We could probably share most of your clothes since your tits aren't that big, and we both have nice big butts."
"Well we differ in the crotch and belly areas, but I don't think that's a deal breaker. I have parts of both genders, it's nice that you would pretend to be like me, even if it's only around the house. That earns you big bonus points," she says as she gives you another big helping of cum. You put your hands over hers on your expanding belly.
"Oohh, that's a lot of bonus points, Cassie. I must have hit the jackpot!"
"No no, you have to jack me off to hit the jackpot. This is just me enjoying myself."
"When are you going to be satisfied? You don't seem to be getting any less horny."
"Why in such a rush?"
"Oh, I'm not. It's just that...if I need to satisfy your boner someplace else, is it going to take this long?"
"Well, hopefully not! This is my first time, I have all of this energy to work out of my system. I need to be able to get off quickly and repeat that also quickly if necessary for our arrangement to work. We can't just disappear to your car and come back hours later with you looking like this! People would suspect...something, if not guess the truth!"
"Haha we could just say we went to an all-you-can-eat buffet and I pigged out!"
"Haha, that's true! I tried to stop you, of course, but you just couldn't get enough, you just had to be satisfied!"
"Ah, I'm thirsty, let's take a break."
"Awww, no way! Why don't you drink my cum instead?"
"I'd love to if you weren't two feet up my butt."
You turn until you have your hands and feet under you. The cock in you was obviously not going anywhere. You push yourself upright, and push Cassie back as you regain your footing. She stays pressed to your butt with one arm around your chest to support you as you make your way downstairs to the kitchen. She pulls you to a stop a few times to cum in you again until you waddle with the weight in front.
"I've never been so horny," she tells you as you drink a glass of milk.
"I wonder if I'm not adding to the problem," you say as you grind into her crotch.
"If you keep being so sexy, I think you are," she grunts as she forces another load into you. "I think I'm not the only one that needs stretchy clothes now."
"Ugh yeah look at me."
"You are gonna have to wear a stuffed bra to make it work."
"So help me waddle to those bags and let's get dressed."
"Kay but my dick is staying where it is."
"Thongs all around?"
"You got it! I'll support your middle," she wraps her arms under it.
You waddle slowly to the bags, but she still cums twice on the way. Once you are there and bending down, she puts her hands to your hips and starts fucking your ass.
"Well if this is a taste of things to cum, I better watch out your cum doesn't start pooling up in my chest, and give me cum-filled boobs to go with my belly."
"Oh don't be silly! It's not like I'm fucking your nipples."
You slip into a high-riding thong as she fucks you. After she cums again, you manage to guide her around so she is between your legs again.
"Want to?"
"Want to what? Fuck your nipples?" She is already pulling out of your bottom with a wry smirk.
Your nipple is hard and slides into her cum hole. She thrusts at your chest without luck; you have no give there. You rub her cockhead to get her off instead. As you thought, any place her cum can go will be able to take it with the safeties in place. You end up watching her dick pump up your chest until, in the space of one orgasm, you have a rather large F cup tit on your chest. Switching to the other, she repeats the process to the same effect.
"I like this, but these look silly with how big your middle is," she says as she cups your breast in two hands and pushes her dick forward, squishing into it. "I love the feeling, I'm more horny than before, I can make these so much bigger," she trails off with her orgasm. After her cum, she deftly swaps sides. Within seconds she cums again. Two nice big orbs that seem to defy gravity are perched on your chest.
"Are these big enough, Cassie?"
"When I stop cumming, we can check," she says as her wonderful balls pump out more.
"Cassie, I'm going to be too front heavy," you tell her after she has given each side two more loads.
"Yeah, you will," she says as she continues only taking a few seconds break between cums.
They are already laying to either side of you, having lolled off of your chest. She can't seem to help herself. Each time she finishes, she looks down at you, switches nipples then starts again. You can't help but think that your belly wouldn't have handled this on its own. As the mountains swell to block your vision of Cassie, all you can do is hope this will reverse itself with time. How will you go to work? No point in going out in public if Cassie already has you looking like this.
"Cassie, uh, these tits..."
"Are great aren't they? So big!"
"They feel as big as my belly!"
"That's about right!"
"Uh, can I get my ass worked instead?"
"Yeah, more ass. Got it!"
Your robot sister turns you over and instead of fucking your butt, puts her cock to your right cheek and cums. You should have realized this before, but the cum seems to pass right through your skin. As long as its under pressure, that's why it didn't go through my skin before when I got a facial, you reason.
So your right cheek balloons out, then your left as she switches sides.
"Now that's a rump! Just a few more."
You can barely move at this point. So you really can't stop her. She is turning you into a blow-up doll. Turning your head after she is done, you see two moons rising high over your back and far to the sides.
"I'm so big, Cassie. You made me so big."
"Let me finish," she says as she moves around your body cumming into you in various places. Hips and thighs on both sides receive her treatment.
"This thong fits much better now," you observe.
"Yea it should, she says wiping simulated sweat from her brow, "Now you should fit in my clothes!"
She slides her dick between your swollen cheeks with a moan, cumming before she even enters you. It runs down to your legs then down those too onto her bed. Then she grabs your hips and fucks you. Your body rocks along with her heavy thrusts. You can feel all of the new weight.