This is my first contribution
I was seventeen at the time. I had gotten home from school ahead of my sister and decided to go into her room and short sheet her bed. My sister and I got along fairly well and we liked to pull pranks on each other.
As I was standing there taking note of how her pillows and stuffed animals were arranged I heard her coming up the stairs. I was surprised she had gotten home so early. I knew I didn't have time to leave her room so I quickly got into her closet and got the by-fold doors closed enough that she wouldn't see me. I was hoping she would leave right away and go down to watch TV, and I would get out of there.
To my dissapointment she had no intention of leaving. She closed her door and pulled a book out of her back pack and layed tummy down on her bed and opened the book to a dog eared page. I soon figured out that it was a book Mom woulden't approve of. One of her friends must have loaned it to her. So I'm watching her through the opening of the closet doors and to my surprise after a few minutes I could see her slowley humping the bed. I smiled to myself and thought this might be interesting. After a few more minutes went by I saw where she had moved her hand down between her legs and she was now humping her hand.
Soon after that she must have reached the end of one of the 'good' parts of the book because she closed it and rolled over on her back. She unbuttened her pants and arched up and pulled them off right along with her panties. I hadn't seen my sister naked for years and was impressed with the bush she had developed. She lay there and slowly started rubbing her clit area with her fingers and then spread her legs farther apart and I could see she was moving her hand up and down rubbing her pussy lips and arching up to meet her hand. I couldn't actually see her pussy from my view point but it was still fascinating to watch her.
By this time I realized my dick was straining to get out of my pants and I knew I was going to have to jack off soon. So I quietly unzipped my pants and let my hard-on out of my fly. I started stroking it and watching my sister do her thing. Soon she had her eyes closed and was making little moaning sounds. She was working her fingers on her clit real fast now and I could see her legs quivering. A moment later she had what I later learned was an orgasm. I watched her as she lay there saoking up the pleasure she had gotten.
By this time I was close to cumming and started looking for something to cum onto. I saw one of her night gowns in the hamper and reached down and got it. I figured once it went through the wash no one would know. What I didn't know was that while I was doing that she had gotten up off the bed, removed her top and bra and was heading straight to the closet.
She opened the closet and let out a shreek while trying to cover herself with her arms and hands. She yelled out, 'What are you doing in here.' Then she looked down and saw my nad still holding onto my erection. She put two and two together and looked at me and then back down at my dick. She said, 'Have you done it yet?'. I said, 'No but I'm close.' She said, 'Well I want to watch.' So I started stroking my dick right in front of her. Soon she said, 'Wait, I want to feel it.' So she dropped her arms and hands away from her breasts and pussy and reached out and wrapped her hand around my dick. My mind was saying, 'This is your sister.' but it still felt so erotic to have someone elses hand on my dick. She started stroking it and I knew the explosion was goin to happen soon. I whispered 'Oh, god, faster' and she instinctively squeezed a litter harder and stroked it faster as I shot my warm cum all over her tummy, almost dropping to my knees.
She stood there looking down at my ejaculate running down toward her pubic hair. To my amazement she stopped the downward flow with her hand and rubbed it all over her tummy and on one of her breasts. Then she sniffed what was still on her fingers and tasted it making a bitter face.
She kind of snapped out of it and realized she was standing naked in front of me and saw her night gown on my other hand. She grabbed it from me and covered herself with it and said, 'I'm going to the shower. This never happened. If you ever tell anyone I'll kill you.'
I've never told a soul until this day. A few years later she asked me what I was doing in her room that day. I told her and we both got a laugh out of it.