My wife gets a call from her mother that my wife's sister is really drunk and she need our help in getting her into bed. Her mother and sister live together and are about a 15 minute drive away. It was after midnight and I volunteered to go over there myself. I get there and I find her sister in the livingroom passed out in just her panties. I had never seen her that naked before and made a comment to her mother about it. Her mother said she undressed her to that point and to get all the looks at her I want. Her mother was irate she was drunk and didn't care her daughter's tits were being seen by me. I didn't mind seeing them but I didn't know how my wife or her sister would feel about me seeing them. Her mother told me to drag her drunken daughter upstairs to bed however I could. I threw her arms around my neck and had her legs sort of wrapped around my midsection with my hands supporting her ass. I carried her through the kitchen, which turned out to be a good thing, because as we passed through it I felt something warm and wet on my crotch. It turned out she was pissing herself and soaking the pants I was wearing. There I was in the kitchen with my pants soaked and her panties fully soaked with a puddle on the floor as well. I put her sister down and her mother told me I better slip out of my wet pants. I did as she asked and was now standing in my underwear, shirt and socks which were also wet. Her mother had paper towels wiping up the puddle on the floor. Once that was done she pulls her daughters panties off and wipes her dry. There I am now standing staring at my sister-in-laws bald bare pussy. The mother hands me the paper towels and tells me I better dry myself off too. The lower front part of my shirt was wet so off it came. My sock were wet too as I mostly likely stepped in the piss puddle so off they came. My underwear was soaked but I was not sure what I should do. Her mother told me to slip them and I think she sensed I was nervous about doing so. She told me not to worry as she had seen many naked guys in her lifetime and no one needed to know outside of us. Off they came and I stood there naked wiping off the piss. Her mother was looking at me and I could tell she seemed very interested in seeing me naked as she didn't take her eyes off of me. I didn't know what my wife would think with me naked in front of her mother but I really didn't have much of an option. Once I was dry I went back to carrying her sister back to her bed. I did so in the same way as earlier which was awkward as her tits were pressed against my chest and her pussy was pressed against my cock. Every step I took my cock and her pussy rubbed against each other. I felt a wetness develop between her legs but it wasn't piss this time. She seemed to be getting aroused and so was I. My cock began to harden and it went from rubbing against her to beginning to poke her at her opening. Her mother was ahead of us as I carried her up the stairs slowly. The poking continued for a few stairs but then I felt my head go in about an inch. If anyone knew I would end up in dead shit. I pulled out but each step caused me to slip back in and it was becoming deeper. We finally reach the top of the stairs and my cock must have been in at least half way. I don't know what happened but with one step, not meaning too, I slipped in almost all the way and her sister lets out a moan. Her mother notices that my cock is somewhere where it shouldn't be. She gives me a look like "I know what is happening" but doesn't say anything. I get to the sister's bed and I lay her down on the bed and there I am with one big hard on. The sister is a little awake now and is telling me not to stop and put it back in. The mother leaves, comes back, hands me a condom and tells me I got 10 minutes. I couldn't resist so I put on the condom and began fucking her daughter while she stayed and watched. It wasn't until after I was done that I realized what I had just done. My mother-in-law witnessed me cheat on one of her daughters and fuck another. Her only reply afterwards was that it looked like I wanted to fuck my sister-in-law and figured I should get something out of it since it was her pissing on me that got me naked. She added she dreamed about seeing me naked but never thought it would ever happen and promised it would be our little secret.