This happened many years ago. I was 16 and my sister was 17. We both worked at a local restaurant and would often carpool to and from work. This particular night after work we needed to water plants and do the regular chores required for housesitting friends of the family. My parents had gone away with friends and my sister and myself agreed to get their mail and such.
After work we drove to tend to the house. After all the chores were done, my sister suggested we quickly jump in their pool to cool down. It was a typical July evening, extremely hot. She suggested we look for swimsuits in the pool house. We were unable to find any, only thing they had were towels. She seemed disappointed so I suggested we just swim in our underwear. She was not as surprised as I thought. With a little prodding, she agreed. She still had her waitress uniform on and proceeded to take off her clothes, all the while telling me to take them off at the same time. I got down to my underwear and she noticed that I had already achieved a massive hardon! This did not stop her undressing. She stripped down to her underwear and bra and jumped in the pool. I followed with a big splash, which pissed her off a bit. As we swam around and cooled off, my sister would regularly go underwater to check out my hardon. She was trying to hide her intentions, but I knew.
As she went underwater one time, I pushed my shorts aside and let my cock flop out. She came to the surface and called me a pig.
As mad as she was, she did not get out of the pool. I thought this was a good thing so I whipped off my underwear and threw them on the deck.
I lay on my back and let my pecker stick out of the water. She was genuinely amused and circled round to where I was. With a sudden splash, she grabbed my cock and started to jerk on it. I was totally shocked and pleased at the same time, this is what I wanted. She looked at me and asked if this is what I had in mind tonight. I smiled and said yes.
My sister then took off her panties and bra and just went wild. It was like she hit a switch and turned into someone else. She grabbed my cock again and started to stroke so hard that I thought she was going to pull my cock off.
I made my way to the side and sat on a step, with her below me in the water. She proceeded to jack me like there was no tomorrow, and with her other hand she massaged my balls. I blew my load so fast, and it seemed like it lasted five minutes. I just kept cumming. When I looked down, she had cum on her face and there was cum floating in the pool. She smiled and laughed like a schoolgirl. I will never forget this event and it was one of many after that night. I will submit other experiences shortly.