I once saw a similar story - the guy who wrote it said it really happened.
A family was in a sauna. They were all naked. Since the sauna was so small and crowded, The son told his younger sister to sit on his lap, to save some space. As far as I can remember, at least the girl was to young for Lit (maybe the boy, too)
At first everything was fine, but after some time, sister was grinding around on brothers lap. The feel and sight of her bare ass on his lap made him horny and gave him a hard one. But the sister didn't stop - instead, she kept on moving - until he blew his load all over her ass. Surprised, she got up and so the parents saw his erected prick and the cum dripping from sisters ass. Of course, they weren't happy about this ...
"We have done so much, with so little, for so long, that now we can do almost anything with nothing." - Dave Marcis