I have seen both my wife's sisters naked a few times, but never had or would have any sexual contact with them. I love my wife and have never cheated on her, but since I was a teenager I have always loved games and over the years have gotten pretty good at transitioning just about any game into a stripping game.
The first time I managed to get one of my wife's sisters naked, it was an unexpected surprise. This is her youngest sister who was about 18 at the time. We were at a family reunion of my wife's family which was held in a forest preserve near her parent's house. There were probably a hundred people there, many of whom I had never met before. So for most of the party my wife and I hung out with her sisters and their cousins and we were all drinking a lot of beer. We had brought our bocce set because that is a great game to play while drinking outdoors.
After about 4 hours the party started to break up. My MIL announced that everyone was welcome back at their house to continue the party and everyone who wasn't already leaving started packing up their coolers. My SIL and I were in the middle of a heated game of bocce, and she announced that she wasn't leaving until we finished the game. My wife said that she was going to go back with her parents and that I should bring her sister home as soon as we finished the game.
SIL and I are both good at bocce and are both very competitive so the game went on for a while, but I eventually managed to win. She immediately demanded a rematch, claiming that she would have beaten me except for one bad bounce. I joked that if she wanted a rematch she was going to have to up the ante and give me a reason to play. She surprised me by saying, "How about we play strip bocce?" I asked her if she was serious and she said that she wasn't worried because she was going to destroy me in the rematch.
We both looked around and realized that the forest preserve was completely empty. It was a huge open area, probably the size of 2-3 football fields, but there was nobody else in sight. On the other hand, it was a public park and somebody could show up at any minute without warning. Despite the danger, I was very excited at the prospect of seeing my wife's 18 year old sister naked. She was totally hot and had a great body, although smaller boobs than my wife. I asked her again if she really wanted to do this, and she told me to get ready to be naked because she was going to literally beat the pants off me.
We started playing and true to her promise she quickly began to run up a score. I lost my shoes, socks, shirt and shorts and she had not lost a single item of clothing. She was dancing around laughing and teasing me that she only needed to win one more point and then I would be totally naked. And then the game turned in my favor.
I won the next toss and she kicked off her flip flops and said I had gotten lucky. Then I won the next toss and she pulled off her t-shirt. We both looked around to see if we had any company, but didn't see anyone. The truth was that her bra looked just like a bikini top, and it was a hot afternoon, so we really didn't look out of place.
I also won the next toss. At this point she asked if I had been hustling her and letting her get a head start, but the truth was that I had been trying to win right from the start. She hemmed and hawed for a minute and I didn't push her, I just gathered up the bocce balls and waited to start the next round. She looked around again, and then said, "Oh what the hell, I'm still going to beat you." And with that she pushed her shorts to the ground.