This took place when I was 17 and my little sister had just turned 13. Because of our difference in age, we really did not have a lot in common. I was a senior in HS and she was in the sixth grade so we had no friends in common or even friends of friends in common.
On this particular day, I had football practice after school and she had a study group after school. Shortly after lunch, the coach cancelled football practice and, unknown to me, my sister decided not to go to her study group.
When I got home that afternoon, being horny as hell and thinking that I would be alone for a couple of hours, I promptly went to my room and stripped. In a matter of moments, I was lying on my bed, looking at my well worn porn magazine and slowly stroking my hardon, building up a head of steam. Of course, thinking that I would be alone, I did not shut my bedroom door.
In the meantime, my sister arrived home and thinking that she was alone because of football practice, went to her bedroom and stripped. She then decided to go to the laundry room to get her clean clothes. As she walked by my room, she looked in and there I was in all of my glory. The first I knew I was not alone was a high pitched 'Kenny, what are you doing to your penis and why is it so swollen and stiff?'
I froze in mid-stroke. I had to think for a couple of seconds before coming up with my response. 'Jennifer, what in the hell are you doing home?' Then I happened to notice that she was as nude as I was and she realized the same thing at the same time. I was very pleased that she was. She always wore very loose clothes and I was under the impression that she was no more developed than when she was as a skinny little 7 year old which was the last time I had seen her nude. I could now see that she was not still that skinny little girl. Her nipples were getting quite puffy and her tits were starting to develop as well. Her pussy was still hairless but, all in all, she certainly was enough to keep my hardon a its full 7 1/2 inches.
I could tell by the look on her face that she didn't know what to do so I looked at her and said, 'I won't tell if you won't tell'. When she started to smile I knew I wasn't in trouble. 'Well' I said, 'first, it's my cock, second, I'm jacking off and third, I've got a hardon from looking at this magazine'. She asked if she could look at the magazine and I said 'sure, come on in'.
She walked in and sat on my bed, right next to me. As soon as she sat down, I layed back down on the bed and started slowly stroking my cock. She asked me if she could help me jack off. I took her hand and placed it on my cock, wrapped her fingers around it and started stroking. I could not believe how good it felt and the fact that my now very sexy 13 year old sister was doing it. I asked her if she knew that girls could jack off too except that in girls it was called fingering. She asked me to show her so I had her stop jacking me and lie down on the bed next to me. I then began to slowly rub her tits with one hand while I spread her legs apart and started to rub her pussy with the other. I alternated between rubbing her clit with my index finger and sliding my middle finger into her pussy. It did not take long for her to come to great climax. I then straddled her pubes while facing her tits and began jacking in earnest. In less than a minute, I blew the biggest load I had ever blown up until then. I then had to explain what I had just shot all over her chest, face and hair. I wiped some cum off of her tits and licked it off my fingers and wiped up some more and put it up to her lips. She promptly licked off and put more on her fingers and licked that off.
We continued to jack off together.