Background:
My name is David, and this all took place during a long vacation for my parents. In the winter of my senior year, they decided to go on a two month long trip across Europe to celebrate their 30th wedding anniversary. Their actual anniversary had been cut short due to the death of a family member and they always vowed they would make up for it later with a large blow-out trip.
I had just turned 18 at the time, but they did not feel comfortable leaving the house completely in my care. This was more out of concern for my safety at being in the house alone for that long, than for fear of anything I might do. They already knew I was sexually active with my girlfriend, having even thrown a huge box of condoms at me when they found out and said "just don't knock her up before you decide to marry", and they also knew I was not much of a partier. But, as I said, concerned about safety over those months, they invited my older sister and her husband to stay and watch the house.
My sister, Ella, was 29 and her husband, Brian, was 31 and they lived much further north than we did, so they were more than happy to be able to spend a month of winter in a much more mild climate. Neither had jobs that required going into an office, and they could remote in from almost anywhere, so it was a fairly obvious arrangement.
The Incident that Started it All:
For the first week or so things were fairly normal. My sister and her husband worked out of the house while I went to school. I had friends over some afternoons, and my sister and brother-in-law gave me plenty of space as long as we did not get loud enough to disturb their work. My parents had let them have the master suite, which was on the opposite side of the house and had its own office space, so we rarely ever bugged each other. In addition, my sister and I had a pretty good relationship, despite having very little in common (I like video games, she was very much a jock). My brother-in-law fell somewhere in between the two of us. He was athletic, but still enjoyed certain games and would even play with me on occasion.
The first incident occurred on the Friday after my parents had left. Late that evening, sometime after midnight, I had just finished saying goodbye to, and seeing one of my friends to the door. We had been up all night hanging, but at this stage none of us were really into spending the night, so they all drove home. With the last person gone, and the house locked up tight, I went to get some milk from the fridge. My usual nightly routine involved a glass of milk and some youtube to wind down and get to sleep.
Now my parents place is a one-story, ranch style house. In its layout, the master suite is just down a long hallway from the kitchen. When I came over to open the fridge, I noticed a light underneath the door to the master, which was shut, and did not pay it much mind. As I finished pouring my milk, however, I heard what could not be mistaken for anything other than my sister moaning her husband's name. It was not all that loud, but the sound was unmistakable.
Curiosity, and some teenage hormones, got the better of me and I put down my glass and crept down the hall. My parent's suite has an old fashioned keyhole in it, one I had used to spy on them, in years prior, and their bed just so happened to directly face the door. I slowly crept down the hall, avoided the boards I knew would creak, knelt quietly in front of the door, stuck my eye to the keyhole and was greeted to a sight I was not quite expecting. Of course I figured there would be some kind of action going on, but I had never imagined that I would get to see my sister's bare-naked ass in front of my eyes. She was bent over the bed, legs spread wide and her ankles tied by a long rope to either foot of the bed. Her long legs, toned ass, and wet cunt were fully on display to me.
For a moment I completely froze, unable to take my eyes off of her. I had seen her naked here and there over the course of my life, usually by accident, but nothing like this. I was soon distracted by movement as my brother-in-law came into view from one side. He was also completely naked, his cock hard and bouncing as he walked, and he was stalking from one side of my sister to the other. He would spank her on one cheek, quite forcefully, move to the other side and spank the other cheek. He seemed to be deliberately slow in his movements, emphasizing and telegraphing each hit, which seemed to be getting progressively harder. Each time he swatted her, my sister let out a moan or grunt, all the while her ass was going through multiple shades of color. He paused the spanking to reach over and rub a finger in between her labia, prompting a gasp and another kind of moan entirely from my sister, like the one I had heard earlier.
They continued this dance of foreplay for a little while. I could not see my sister's upper body or face, being that she was bent over and I imagined she was tied to the head of the bed as well, but I could tell she was getting more and more turned on. Her moans became increasingly desperate, and her pussy was so wet, there were actually some drops on the floor. Finally, she appeared to have had enough and I heard her demand "dammit, just fuck me already!". Her husband climbed onto the bed and apparently grabbed her by the hair, as I was able to see his hand and the top of her head. He whispered something into her ear that I could not make out, but I could guess as she soon said, in a more submissive voice "Please fuck me, Sir. Fuck your little whore". He replied with a "good girl" before climbing back off the bed, positioning himself behind her and, by the sound of her moaning, entering her hard.
Now my view was of my brother-in-laws ass and the rapid movements it was making as he fucked my tied up sister. I am bi, and he has a very nice ass, but I have to say this was not as interesting a view as the previous one. Nonetheless, my sister's moans were so hot and I was as hard as I had ever been. My brain was in overdrive trying to parse and rationalize it. I was not thinking of the fact that she was my blood sister, only of how hot she was, and of how hot she sounded. Additioanlly, I was thinking of my girlfriend, the way she moaned, and how fucking hot it would be to have her tied to a bed like my sister currently was. All of this sent me into a horny overload and, against my better judgment, I slipped my hands into my pants and began stroking myself right there in the hallway.
It was not long before my sister's moans reached a crescendo, high enough that I thought I might have even heard it from my room, and not long after my brother-in-law moaned for his own orgasm. Moments later I myself came. I kept myself silent, a skill I had learned early on as a horny teenager, and made sure to drop my seed into my own pants, since I can just throw those in the laundry. As my brother-in-law untied my sister I began to pull up my pants and prepare for my stealthy exit, but then she did something that recaptured my curiosity.
As she was freed from her bonds and stood up, she took her husband by the hair and, with a devilish grin, said "my turn". I watched as she pushed him onto the bed, fetched more rope, and began binding him up. She did not tie him to the bed as she had been, instead she used her ropes to tie each of his wrists to a corresponding ankle, forcing his face into the bedsheets, and his ass and cock high into the air. Finally she retrieved a ball gag saying "I know how loud you get when I do this" as she stuffed it in his mouth and buckled it behind his neck.
After completing her setup, she stepped to another side of the room I could not see. I was left, for a moment, with a view of my brother-in-law trussed up with his ass in the air and a hardened cock fully on display. Despite my recent orgasm, I began to become turned on again, not only from the sight itself, but from imagining myself in that position. I had seen it before, in a Hustler Taboo magazine that featured femdom. All of this was something I had dreamed of doing with my girlfriend, but we were both a bit nervous and neither of us owned any equipment to make it happen.
