Found on a online forum. The guy and his sister had sex when they were single, then both got married and they end it, until years later, his sister got separated from her violent husband and the guy's wife invited her to stay with them at their place.A very bad idea! A week after his sister moved in they started doing it again. The guy says the best time is when his wife is working in the night shift, they can do it everywhere in the house and not being afraid of being caught.