I read it a while back. It was about a brother and sister in the car with the rest of their family. Surrounded by luggage, and her sitting on his lap (because of the crowded car) he accidentally starts having sex with her because of a bumpy road and his dick slips out of his swimsuit (they’re going swimming). Title had Cabin in it. I think.
Source: reddit.com/r/Erotica/comments/a9s459/cant_find_inc_story
I read several versions of this story, brother – sister, mother – son, on the way to a cabin, on the way to college. Have you tried to search for ‘car’ or ‘backseat’ here or maybe on Literotica?
“Love at first bump “ literotica?
https://www.reddit.com/r/incest/comments/4o0ls4/bs_long_drive_with_sister_on_my_lap/?st=JQ5Z01PP&sh=5def8f9c
This one?