I too have fantasised about my mother in law, now ex mil. She had a very nice shape. One day I took a last minute day off from work. After the wife left for work I got out a couple porn videos to jerk off to. I was sitting in the living room, naked enjoying a bit sexual 3way when I heard something hit the floor in the kitchen. Thinking it was one of the cats I went out to see what happened.
There she stood, dropping off some canned peaches. She had dropped one on the floor. So there I stood...naked with a dripping boner. She looked up at me while I tried to cover my cock. She smirked, averted her eyes, sort of, and said, "you go back and finish what you were doing, I'll stopped back in half an hour?"
All I could say was okay. I don't think she was even out the back door when my cum erupted from my cock. Kind of wish that had happened while she was standing there. She never mentioned the incident or even gave any clue it ever happened.