Blame it on Rio
Starring Michael Caine and Demi Moore
Directed by Stanley Donen
1984
This is another historical sexual oddity. No way would this ever get produced today, save perhaps something similar in the adult genre. In one of her first roles Demi Moore plays 17 year old daughter to Michael Caine - both go on a vacation to Rio with their mutual best friends, another father/daughter pair.
Caine and his best friend's daughter seduce each other. Hillarity ensues, as one might imagine it. Movie includes a fair number of topless shots, but alas, not of Ms. Moore.