If I recall correctly, the story had 4 characters: a mom, a son, and two daughters. They get shipwrecked and have to resist their urges to fuck. They start with simple sex acts like handjobs but the mom refuses to let them do vaginal sex. Yet, as their barriers to taboo break down, eventually they do start sneaking off to have sex without the others knowing. They start to show with his children and they realize that they had all been fucking so they don't keep it a secret anymore. The story ends with them getting rescued and giving birth when they return to society.
Some additional details I remember are that there was a heavy emphasis on describing the women's bodies as they changed, and I'm fairly sure there were fruits on the island that increased fertility and increased breast and penis size that broke the dam open for them to start having sex in the first place. The pregnancies made their breasts grow even more and the author loved to describe it by giving you their cup size.
I believe this story was on Literotica, but there's a chance it was on something else like StoriesOnline. I would have read it circa 2019, and I don't believe it was very old at that point. Does it ring a bell to anyone? There are so many deserted island stories but I can't find this one anywhere and it was my favorite by far.
