About 15 years ago we built a sauna and hot tub addition off our bedroom, for use in our freezing northern winters. My wife's older sister and her husband, sometimes come to stay with us for long weekends. They both like our sauna and hot tub and have used it naked before. We have camped together and used portable outdoor showers, so we have seen each other naked before. That night we went to use the sauna. My wife and our brother-in-law can only stand it so long in the sauna, so they are usually the first to leave and cool down before we use the hot tub. This night my sister-in-law stays in with me for a long time, shifting a couple of times and giving me a great view between her legs. She has larger, firmer breasts than my wife and has more hair around her pussy, otherwise they are similiar. I am becoming aroused but not totally hard. We finally come out to cool off and I jump in the shower to cool down. Whilst soaping up I notice that I am quite firm, about seven inches, but not totally hard yet. I towel off and join the rest in the hot tub. We had planned to watch an x-rated movie after the hot tub but my wife said that she was too tired to join us. I get out and go for a pee. When I return my wife is in the shower and her sister is playing with her husbands rock hard six inches. This could only get better!! My wife comes out of the shower, and goes to bed. The rest of us go to the family room loosely wrapped in towels, turn down the lights, and start the movie. I don't remember much about the movie but boy! do I remember everything else. The in-laws sat on the sofa and I sat in a recliner. The movie was playing and so was my sis-in-law with her husband's cock. He looked like he was fingering her so I thought `what the hell` and started to stroke my already half-hard cock, watching them more than the movie. In a couple of minutes I was rock hard. I'm more of a grower than a show'er. I don't think she had ever seen my cock totally hard and I could see that she was watching me AND the movie. In a few minutes he was coming hard, fortunately she had a paper towel to clean up. I wanted this to last forever, and you can imagine my surprise when she came over and helped me out too, stroking my now very hard cock. I couldn't hold back much longer, and knew from the length of time that I had been hard or half-hard that I would cum really hard!! I shot several long sprays, the first hitting her breasts and the rest on her stomach. She continued to milk cum from my cock as I laid totally exhausted. After that we went to bed. My wife was sleeping naked with no covers on, so before morning she got serviced as well. I felt like I could cum all day, the next day. In the years that followed, the four of us had some more adventures, but that will have to wait for another day. Thanks for reading!!