I felt guilty about this afterwards as it was my sister that was involved.
We lived in an old farm house which my parents were renovating. In one corner of the expansive back garden area was a very large old wooden building which father used as a storage area for stuff which my mother thought should get thrown away. Lol.
It had seen better days, the roof leaked in places, and in one corner where it was set into the rising ground behind, it was starting to rot through completely.
My sister, two years older than me, had started dating this local guy. He was no hulk, he had a build more like that of a horse jockey. But he had a great sense of humour, was fun and always seemed happy and cheerful. He was also deceptively strong and fit - something even my father noted when he helped with the renovation work.
One summer evening, I was walking home from my friends house which was pretty much across the field from ours, and took my usual route across our land and down towards our house. This took me past the rear of our old building, particularly past the corner spot where it was let into the (higher) ground behind. This in effect meant that I was almost at roof level as I approached. I can't say what alerted me, but as I approached I was aware that something was going on in there, so tip-toed as quietly as I could to see what was afoot.
A couple of smallish holes in the roof meant I could see in as long as I was careful and balanced myself on the edge of the ground and didn't lean too much against the rotting timber sides. My sister and her boyfriend were sat on an old garden bench which father had stored there, and were embracing one another with some passion! It was just about dusk, and the light was fading fast, but there was just enough light for me to see that she had her breasts uncovered and he was fondling them and kissing her nipples and her lips. What riveted me to the spot however was the sight that she had one hand on his crotch and was clearly rubbing her palm over him. I knew I should have left, but the sight just seemed to hit a spot with me and I realised almost immediately I was becoming aroused.
After a short while he got his hand up her skirt, and I saw her part her thighs and even heard a slight gasp as his hand found her. I can remember thinking 'OMG, he's fingering her!' and feeling my own excitement mounting at the thought. There followed some fidgeting and repositioning as they both found a more comfortable position, but that meant that whilst I had a better view of him, I couldn't really see what he was doing to her. I did however see that her nipples were very hard and erect and seemed to be very prominent, and that her legs were now wide apart and her skirt hiked up.
Then, she started to undo his trousers and helped him slip them down just enough to get at his pants. Then ... wow! .. her hand went under the material and I felt my heart pounding and my pussy twitching as I imagined how it felt for her. I was sure they must hear my heart pounding or my breathing so found myself holding my breath, hardly daring to breath or move. When she got his cock out I was totally blown away. It looked so hard and large, and very thick (but in retrospective it was probably not that large). I heard him moan a little as she started to wank it.
It looked so lovely and sexy seeing her hand gripping his shaft and seeing her wank it that I was fighting the impulse to touch myself despite being aware just how aroused I was, fearful of being discovered if I slipped or made a noise. They then moved, she sat astride his thighs - one leg either side of him and up against his cock facing him, her breasts still exposed and her skirt up so that her knickers and crotch were visible. For a moment I thought she was going to sit on his cock and they were going to fuck, but instead she sat like that and continued to wank him.
She was looking at his face, smiling and pulling sexy expressions as she did it. His stomach was exposed as his shirt was open and I could see his stomach muscles working as he enjoyed the sensations of her touch. I then lost my own battle, undid the top of my jeans and let my hand slip down and onto my pussy and start to rub my clit doing my best not to slip or make a noise. I think I realised immediately that I would cum pretty quickly so tried to hold back. A couple of times he reached up to play with her breasts obscuring my view of her hand on his cock. And also a couple of times he reached forward to feel her pussy, and I guess play with her or finger her but his body obscured my view as to what he did exactly.
Whatever it was, it solicited gasps, wimpers and body movements which looked really sexy from my sister. But the fact I couldn't see the detail didn't matter - I simply enjoyed the view and imagined what it felt like to be in my sister's shoes. When he leant back and raised his head so he could look down at my sisters hand working on his cock, I saw his stomach muscles working over-time, and I heard him say '.... coming, slower, I'm coming' And my sister saying 'oh yes, go on, go on' and then heavy breathing and gasping exhalations and his cock started ejaculating.
The first lot shot right up in the air and my sister let out a loud 'Oh!' as the next spasms sent it all over her hand, arm and his body. That was too much for me and I had the most intense orgasm with hardly a touch on my clit to bring me off. It was all I could manage to remain quiet, or stationery as my legs seemed to wobble and threaten to give way.
As soon as I was able, and had recovered enough, I tip-toed away back to our house and immediately to my room where I did it to myself again, and again came very quickly and intensely with the images of what I'd just seen still fresh in my mind.
I felt a bit guilty the next day as it was my sister that was involved, and that seemed a bit pervy to me back then. Still, I'd love to hear from anyone else who has had similar experiences or who simply wants to share their sexual experiences. BTW, I'm bi, something I discovered shortly after the above incident.