I was doing some work at my mother-in-laws house. She said she was feeling tired and was going to take a nap while I continued to work. A little while later I needed to use the washroom. I went in and pushed open the door to find her in there topless in just panties washing her hands. She quickly covered herself and I quickly left but not before I saw her breasts. She came out and we had a talk about the incident. She was still topless in panties covering her breasts with her arms and thought it only fair that she see me completely naked. I did see her breasts, and only her breasts, for a couple seconds but she wanted to see all of me. I mentioned how little I saw of her so she agreed that I need only strip to my underwear. I stripped to my undies and she said if I had caught her fully nude that my undies would be off right now. I said that may have been the case had I seen her fully nude. She put her arms to her sides fully exposing her breasts to me which caught me by surprise. She walked over to me, pulled my undies down and stood there looking at me. I was just standing there naked letting her look as I was looking at her breasts. She said I saw as much of her I was ever going to see but I didn't have to get dressed if I didn't want to and walked away.