Was perusing an old copy of the Decameron by Boccaccio which is similar to the Canterbury Tales by Chaucer. People often outsmart themselves in those tales and it reminded me of a story with a comparable ending. You'll have to read it to compare.
A girl tells the story of her life in remote Wyoming, and how she and her brother learned about life and having babies together. Mom and Dad discover their relationship and while trying to discourage it get drawn in. / (Reviews)
A mother and daughter switch bodies. The premise of this story is from the movie Freaky Friday, which I only saw the trailer, but I am sure is nothing like this. I would be interested to get feedback if you consider this incest or not.
21 year old Lynda and Kelly are identical twins. That is unless you count Lynda's 12 inch cock! Kelly, infatuated with her sister's penis, intended just to blow her, then rub her wet pussy safely along it's generous length. Through a mischance on Kelly's part, she accidentally takes Lynda all the way inside of her! Virgin no more, and with her libido fully awakened, Lynda fucks her panicked fertile sister all over the house! Things get wilder when their lovely drunken mom comes home in part 2!
Boy starts getting lucky with his girlfriend, and is thus observed. So boy starts getting doubly lucky, with his older sister. They of course both grow up and go on to have separate lives. But still they intersect. As always, enjoy! My apologies of course in the endnote.