Last summer, my family and I were at a water park with my wife's other siblings and their families while vacationing together. We all went to line up later in the afternoon to go on the go-kart rides there. Somehow, I ended up at the end of our group of seventeen with my fairly-attractive sister-in-law. It was really hot and sunny, and she asked me if I would rub some suntan lotion she was holding on her shoulders. At first, I didn't think anything of it and obliged, but as I rubbed it in, I was getting turned on more and more. I started to rub slowly under her swimsuit's shoulder straps. She just bowed her head and let me keep rubbing slowly all around her shoulders. Suddenly, a few of the kids in front of us were horsing around and somehow bumped into my sis-in-law who, in turn, bumped back into me. Being erect, I lucked out in landing it in a perfect spot between her butt. She was startled but didn't say a word or even move. She didn't even turn and look at me. I just kept rubbing away not moving at all. She didn't move either. Every once in a while, I would slowly push a little closer. Again, no movement from her. It was amazing. No one in front noticed, and no other people came and stood in line behind us either the entire time. Everything fell into place perfect literally and figuratively. She and I have never been on flirting, let alone entirely friendly terms since her husband and I do not get along well. Besides, she always seemed devoted to her man in every way. As we were leaving the park, she never did say anything to me about it, but she did flash a little devious-like smile as we all got to our cars. I immediately resumed my erection seeing that. I've decided I am definitely going to pursue something further with her.