Author's Note: the following story is entirely fictional.
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My wife told me that her sister was coming to visit us for a few days. She recently took a new job and would be heading cross-country for several months of training. My wife's family lived in the next state over, so we hadn't seen them very much. It had been about 2 years since we'd seen them last, not since her sister graduated high school.
My wife and I are both in our early 30's and have an active sex life, but sometimes the stress of the day wore us out and we ended up falling asleep without enjoying each other.
My sister in law flew in by plane and my wife picked her up from the airport. I had to work that day so I didn't see her until I got home. The first thing that I noticed was that her baby sister had definitely grown up since we'd seen her last. She was about 5'6', maybe 130 pounds. But, she sported a very healthy set of D cups and her ass and legs seemed more shapely than before. She had definitely kept herself in good shape.
That first night was a lot of a catching up. We went out to eat and then came home and sat around on the couch and talked. We have a L -shaped couch. My wife sat in the corner and her sister sat on her left, and I sat on her right. We have an ottoman footrest and her sister had her feet propped up on it showing off her shapely legs. She was wearing a mid-length skirt. I kept watching between them, but my eyes always seemed to find their way back to her legs, they were just very sexy.
At one point my wife's sister said something and I looked up half-startled. I had been staring at her legs again. When I looked up, she gave me a sly smile and a wink and kept talking like nothing had happened, but she knew! She had caught me looking and said nothing.
On the pretense of getting more comfortable, she tucked her right leg under herself in a sort of half-cross-legged position. My wife could see nothing unusual from where she sat, but from where I sat, it opened up her legs and allowed me to see the pink of her panties up under her skirt.
She teased me like this for the rest of the night. Eventually, my wife fell asleep on the couch as she usually does. Her sister and I talked for a few minutes more before she announced that she was going to shower and get ready for bed. She was in the main bathroom at the top of the stairs. From the bottom of the stairs I watched her ass as she disappeared into the bathroom and closed the door. I could hear the shower going and I got hard picturing her naked just on the other side of that door.
Eventually I heard the water turn off. I waited a couple of minutes and came upstairs on the pretense of turning down my sheets and getting ready for bed. As soon as I got to the top of the landing, my sister in law opened the door to the bathroom. She seemed surprised to see me, but smiled a devilish grin. She was wearing one towel around her head to dry her hair, and the other towel was wrapped around her body. The top of the towel was just above her nipples, showing some decent cleavage. The bottom of the towel came down just a couple of inches past her crotch. She was carrying her clothes in her arms and dropped her panties when she got to the door of the bedroom. She bent over to pick it up and I could see the bottoms of her ass and the folds of her pussy tucked away. She lingered for a moment, pretending to have trouble picking it up, before standing and smiling at me over her shoulder. She sashayed into the room and turned and closed the door. I've never seen a door closed seductively before, but she certainly managed.