Obligatory: Details changes slightly for an attempt at anonymity.



I've known my sister-in-law for over half of her life, having met my wife as a teenager and her sister being the youngest of 6 siblings. When we first met, she was always the adorable youngest sibling, cute, bouncy, full of spunk, and loved attention. Those attributes remain relatively unchanged, today (11 years later), but for the addition of newer, more sultry ones: flirtatious, incredibly attractive, and intelligent.



In our teenage years, I would spend all of my free time with my wife-to-be. We would often end up back at my future wife's house for the evening, and if we weren't sneaking around in the basement trying to secretly and quietly make each other climax orally, we were, in general, spending time with her family upstairs, where the sister (let's call her Mary), would surely stick closeby.



Mary and I developed a closeness, which stemmed from my having only younger sisters myself, and my affinity for her assertivness, which is quite the opposite of my own personality. If we were all playing a board game, or cards, and if there were teams, Mary would always end up on mine; if there weren't teams, she would always sit next to me. It quickly became known and accepted that we were buddies, and she became known as my "favorite sibling" among the others. As we aged, the friendship between Mary and I also matured. She would date guys, and break up with several boyfriends as she went through high school herself, and would always come to me for advice, or merely to chat about her life and goings on. Having established a long friendship already, this was not out of the ordinary.



Given my wife an I are from the same hometown, in my college years I would return home for summer, and often return to my wife's parents house, with my wife. It became acceptable to me to stay over from time to time (albeit in the basement, far from the girls' bedrooms on the second floor), mostly because I had become a staple in family life. All of the siblings and I get along; but Mary and I remained thick as thieves.



I proposed to my wife-to-be and my parents divorced when I was in college. I needed a place to stay, and the obvious choice was with my wife's parents, until I found a job and was able to buy a place for my wife and I. My wife found a job early on, and began building her career while I was finishing up a masters. Meanwhile, I would often find myself alone at the house with Mary. She turned 18, and began to dress the part. She had her friends over, and generally did what 18-year old's do. I focused on school, job prospects, and merely kept an eye on Mary, stealing glances when I could, and engaging her when appropriate.



Mary began to attempt to integrate me into her friendships. All of her best friend's knew me, and all of her boyfriends seemed to look up to me as some sort of standard against which they were being held. She compared them to me, to their faces, and every one of them made a substantial effort to earn my friendship and trust. All of this behavior was platonic, and mirrored the way a younger sister would treat an older brother. Slowly, Mary started to let her guard down.



It was clear to me, that Mary had developed a crush on me, or at least an interest. She would start to wear pajamas around the house until noon, leave doors open when she showered, and make semi-innapropriate and suggestive innuendos to me. We developed inside jokes, habits, and more closeness. If it was open, Mary always took the seat next to me on the couch for a movie, tv, or other activity. She cared less about personal space. Her legs often brushing mine. Sometimes she lay her head in my lap (I struggled immensly not to get hard, and sometimes I did - if she noticed, she never let on). If I came home from an internship, she would, without prompting, massage my back, feet, etc. If I had to run to the store, she would volunteer to come with me. My wife - if she was around - would laugh, sometimes roll her eyes, but never gave it too much thought. I always enjoyed the attention.



Fast-forward to today. The sexual tension seemed to compound over the years. Our relationship toed the line between platonic and semi-platonic. I would often find myself thinking about Mary and what she might look like naked, wet, and with me inside of her. Does she get wet when she thinks of me? She would frequent my fantasies, appear in my dreams, and almost exclusively the source of my jerk-off material. Her sex drive seemed much higher than my wifes, and often, it seemed I was the focus of her attention. Everytime we would meet, at a family event, or family vacation, etc. Mary would embrace me first, before anyone else, and always. Where sometimes she would hug her other siblings, or other in-laws, (half of the siblings are now married), she would never miss an opportunity to grab me, frequently with a spring in her step as I arrived with her sister. She would meet us at the car, help unload, and be, generally, friendly.



In June, we went on a family vacation. Mary had just broken up with another boyfriend. She was especially distraught about this one. All the siblings rented a house on a lake, with too few bedrooms, but enough space to sleep us all. Typically, when we all get together, we arrive on and off throughout the week due to mixed schedules, etc. So, the single siblings will sleep in beds until the married siblings arrive, and then the singles will give up the room and move to a pull-out sofa, air mattress, etc.



My wife and I arrive and are assigned to an upstairs bedroom. This house has no a/c, but we're on a lake and it should be fine. The first night is hellacious. The heat upstars is so bad, I open the windows, crank the fan, open the door, and strip everything off, sleeping naked on top of the sheets.



The next day, Mary and I hang out in the lake on a raft. She notices my new designer sunglasses and comments on them. She takes them off my face and looks at them, complimenting my choice, the fit, and the shape. She suggests she should order matching ones when she gets back inside, because "then we could match." I blush and agree. The other siblings mind their own business and catch up on the beach, or on the porch, etc. Drinks go down easily. We find ourselves alone, all of the rest of the siblings drifted back toward the house. I decide to lay flat on the raft. Wet bathing suit clinging to my body. I usually am super consious about avoiding any appearance of improper behavior around Mary. Either the drinks or the heat make me not care today. I let the suit cling and outline my cock. I am larger than average 7.8-8.2 inches (depending on arousal), and in the heat, everything is loose and plump. My cock and balls are clearly visible to Mary. I catch her looking, which makes my cock fill, but not rise. eventually, she catches herself and excuses herself to get us more drinks.



My wife and I go upstairs to change. We enter a bathroom, door open. The only room down the hall is the one we are staying in, so there is no reason for anyone to walk by. Unbeknownst to me, Mary decided with my wife to stay in our room tonight, becuase there is a small, but available bunk bed. She decides the top bunk might be more comfortable than the couch. I am standing buck naked with an open door, my wife has slipped into the shower. I am turned so that I cannot see the hallway, but the hallway can see me. Mary walks by, says "hey" casually, and dissappears into our room. It was impossible that she did not see me, full frontal, cock and all. My wife laughs from the shower and jokes at me that her sister saw me naked.



That evening, Mary decides to sleep in a bunk bed in the same room as my wife and I. She takes the top bunk, because why not.



We partied late into the night, and it was time to turn in. As we were all a bit tipsy, I was more than a little horny. I threw caution to the wind, entering after my sister in law and wife were already asleep and stripping naked in the middle of the room as I did the night before. I knew Mary wore contacts, and I know I can't see very well without my glasses, so I figured at best, she would see a blury naked figure, if she woke up; but the allure of being naked in a room with Mary was too much to ignore. That night was even hotter than the one before, and thoughts of Mary swarmed my head. I could hardly sleep with the heat, and knowing Mary could make out at least some of my nakedness if she looked (she had a perfect view from the top bunk), made it even harder to get some rest. I pretended, as best I could, that I was asleep. I was 80% hard all night. I would turn to accentuate my cock, as it grew along the length of my forearm. Finally, i turned and slept on my back, telling myself to let her see a bit more, if she was awake. I heard stirring from the top bunk. Movements were made as if Mary was taking down pajama pants that she probably wore out of concern for sleeping with us (I know she sleeps naked at her house, because she sometimes video chats after waking up, and has made sure to point out her nakedness under the covers). She never removed her blanket. I thought I saw her peering over the edge of the bunk, but my view of her was not as clear as hers would've been of me. I started to get harder. It was a split second choice, to either cover up, or let it happen. My cock rose, my arousal clear, to full mast. My balls hung loose, yet still managed to produce ample pre-cum. I casually moved as if asleep to "tuck" my cock under my arm. That's when I felt the slickness it had created. I, as casually as possible, brushed the liquid along the length of my shaft and around my glans, getting harder. I heard motion that was every so lightly distingusihable from the full-speed fan. I imagined that it was the sound of a Mary running her fingers along her wetness. The slight sloshing sounds of her moving from her clit to her open legs. I saw her legs spread, but not the motion; she seemed still from my angle. I was now openly stroking, becoming slicker. Edging my way through 2 partial orgasms, allowing myself only to dribble a small amount of cum before using that droplet as more lube for better stroking. My wife stirred. She half-asleep told me I should consider covering up because Mary was in the room; but she turned to go back asleep. I heard what I thought was a moan. I managed a few more strokes before I shot my cum, in 8 large spurts, all over my chest, each shot arching high before landing with an audible splat. Mary noticable turned to face the wall. I drifted to sleep.



In the morning, I prentended to be asleep until I was the last one in the room. Mary, before leaving, made motions that looked like she was putting on her pants under the light blanket, then she promptly left.



No one said anything about that night. Mary treated me like always, flirty, but no more so. I logged the memory as a risky, but satasfying moment, still unsure if Mary saw me, or if the fan noise was playing to my imagination.



Only later, after a week or so, did it hit me. Mary is far-sighted.