Thanks everyone, I've requested an Ao3 account and will post the story there once I get it. I will probably post a more sanitised version on literotica, where all the characters are over 18.
In the mean time, here's a short excerpt from the 2nd chapter, after the main character learns he has impregnated his twin sister:
“I’m doomed to end up looking like that aren’t I,” my twin sister said, looking up at the pregnant stone figure with its huge boobs and gravid belly. Nine months felt like a long time, and I wondered just how big Sara would be by then.
I sat down next to her. “I think you look a lot sexier than her.” I said, smiling.
“Will you still think that when I’ve got a big fat baby belly?” she asked, looking back at me.
“Damn right.” I said.
“Prove it.”
I slowly lifted Sara’s tank top off of her, struggling a bit to get it over her breasts. They had indeed grown bigger over the past few weeks, and were now as big as Mom’s had been during my “physical” all those weeks ago. I took both two handfuls and started to knead them and play with her nipples.
Sara pulled my t-shirt over my shoulders, exposing my still-pale chest. “You’ve definitely gotten more muscular.” Sara said, eyeing my body critically. I hadn’t noticed, but then again I hadn’t exactly seen myself in a mirror lately.
She turned her attention to the fat bulge in my pants, and slipped her hand under my waistband.
“Jesus, just how big are you now?” she asked me, struggling to get my huge dick out of my pants. In the end she just pulled my pants down instead, freeing my cock to spring upwards and almost hit her in the chin.
“11 inches” I said. “Maybe slightly more, it’s getting hard to tell.”
“Fucking hell.” Sara put her hand round my cock, but there was no way she could encircle its girth now. I put my hands on it too, above and below hers, and between the three of us we barely covered its full length. Sara cursed again, under her breath.
“Want to fuck me again?” she asked, after a few minutes of slowly stroking my cock.
“God yes.” I replied, and she lay down on her back with her legs open. I slipped her panties down her legs and put my head between her thighs, taking the opportunity to eat her out first.
“At least we don’t have to worry about pulling out any more.” she said, as I tongued her clit. “It’s not like I can get more pregnant.”
I sat up and looked at my sister. She lay there, legs apart and snatch on display, completely at ease at being naked in front of me. I placed the bulbous head of my cock against her slit, and marveled at the sight of my nearly-foot-long dick about to enter my own sister’s sopping wet snatch.
“Now, fuck me bro. Fuck your pregnant sister.”
I slipped it inside her, gently this time, giving her the chance to adjust as I slowly filled her up inch by inch. Eventually, finally, I got all 11 inches inside my sister’s cunt, my balls resting against her ass. We both took a breath, locked together with her legs wrapped round my waist.
“What will Mom say when she finds out?” I wondered, out loud. Sara hushed me, guided my hands to her huge tits, and I started fucking my sister’s pregnant pussy with my massive cock.