The first time I had this experience was about twenty years ago. I made sure to repeat the experience again after that. I even told my wife the other day!
Just after we got married we stayed over at my inlaws for a week. My father-in-law was already asleep. He never stayed up late, and my wife was sleeping after a long trip in the car during the day. I was just finishing my coffee we were having while chatting when my mother-in-law asked to be excused.
She said that she was going to take a quick bath, as she was sure that I would like to bath before going to bed. I said that I would and busied myself with a newspaper lying on the table. I heard her close the door to the bathroom and the splash of water as she was running a bath. I touched my still soft dick through the shorts I had on. It responded quickly. I could feel it stiffen beneath the soft fabric. I kept an eye on the bathroom door to make sure I wouldn't be caught having a wank in the lounge.
I was hard and had to take out my six-inch cock to have long comfortable strokes. I imagined my mother-in-law in the bathroom, naked, lying back in the water. This vision had me speeding up the movements of my right hand.
I noticed that the bathroom had a glass louver above the door. I thought of what I would see and softly got up. I went to the bedroom on tiptoe to make sure that everybody was asleep. Both of them were snoring so I looked for something to stand on. My eyes caught a side table in the dining room. I picked it up and softly put it in front of the bathroom door. Everything was quiet. I couldn't hear a sound coming from inside the bathroom either. I stood up straight and peeked between the glass slats. I had to stand on my toes to get the right angle of vision but it worked.
First I saw the bundle of clothes in front of the bath. Her panty was lying on top and this got my cock jerking again. I altered my stance and looked into the bath. There she was! Laying in the bath with her head on the side. Looked like she was sleeping but for a hand moving the water ever so slightly at her side. I started wanking again.
I looked at her boobs. They were much bigger than my wife's. Her nipples and areola were similar though. She was unshaven and the thick patch of hair was sticking out above the water. The hair moving in the current she was stirring with her hand. Her feet were up against the other end of the bath, legs together. I focused on the hairy bit while quickening my pace on my dick.
Suddenly she sat upright and reached for the shampoo on the edge of the tub. I nearly had a stroke with fright of the sudden movement. She proceeded with washing her hair.
Watching her boobs jumping up and down with the motion of her washing I started cumming. My cum spurt against the door and started running down. I came more than when I normally wanked and enjoyed it for as long as possible.
I got down from the chair and hurried to put it away. I wiped the door with a napkin I found in the kitchen and went to sit with the paper again.
When she came out of the bathroom I got a hard-on again seeing her in her robe. She said goodnight and retired to her room.
I had a wank in the bath again and more than once made love with my wife imagining lying on top of my mother-in-law. I have at an occasion caught her playing with herself so my fantasies concerning her are quite detailed.