I have done a few things with my wife's mother; mostly tame compared to other stories here, but could also be pretty wild by others' standards, I guess. I have posted all of it here in the past, so you can check my posting history for ALL the details, but here is a MUCH shorter summary for the newer folks...
First time she saw me naked was when I had a newborn in the shower and she took him from me when I was done. I stayed in the shower while she put the baby down, then she came back and watched me while chatting casually, then when I was done I watched her take a bath. Just casual conversation, but we stared at each other, certainly. She was in her very early 60s at the time and had the body of a perky 25yo--pretty amazing, actually.
We have seen each other nude too many times to count on trips while sharing hotel rooms and cabins. She stares at me like it's her first time seeing me every time. My wife just giggles to herself.
She walked into the room of my wife and me one morning on a cabin trip to ask a question without knocking. She saw my full morning wood while I was standing next to the bed. I didn't cover it up, and my wife's sister came by to see what we were talking about, and her sister bolted. My wife was upset at first, but then it turned her on.
I have been skinny dipping in front of my mother in law many times.
But the "big one" was probably close to a decade ago now, but she was at our house while I showered during the day after cycling. I won't write all of the details again, but it lead to touching and other things, and perhaps she had a big O.
Due to moving, growing families, and other nuances, we don't travel together or spend as much time together as we used to, so nothing has happened for several years now except a month ago she came over while I was in the shower (clear front door, straight view into the bathroom, clear shower door), and she waved at me from the front door while I dried off, dropped something off on the step, then left.