This starts off when I was 16 years old and so was my sister. We are both moderately athletic and in shape. She is really good looking with a nice ass and chest. I've always thought about doing something with her mutually but never knew if she had the same feelings for me.
When we were 16, we often stayed up late together watching TV when our parents went to bed but this was normal. Though one day she was sitting on the couch across from me and was wearing a tank top with no bra, and short shorts. I was a little bored watching TV and my mind started to wander and I was thinking about how she would look naked. But because of this I quickly got a raging boner.
I was wearing shorts and my boner was visible but I had my knees up so she wouldn't see. I guess she was thinking about the same kind of thing because when I looked over at her I could see that her nipples were hard and she was discretely rubbing between her legs over her shorts. She wasn't doing it very much but I could tell what she was doing.
This turned me on so much and I worked up the courage to say 'Having fun?'. She looked at me and looked like she was nervous and scared that I had caught her. I told her that I was horny too and also asked her if she masturbated. She said that she had done it before but not in a long time and had never had an orgasm yet. I was so horny at this point but didn't care so I put my legs down to let my boner point straight up in my shorts.
She looked at it and was shocked. My dick is only about five inches hard but she said it was the first time she had ever seen one even through clothes. I told her that I masturbate every day and I started to pull my shorts off to reveal my bare penis to her. She looked too horny at that point and when I looked down I could even see a wet spot in her crotch area in her shorts. I started slowly jerking off in front of her and she removed her shorts and started rubbing herself.
For only being 16, she had such nice breasts and such a nice vagina with a little hair on it. She was rubbing herself so fast I could hear the juices squishing inside of her. This made me so horny that I could not go any longer, I pumped so much faster and blew the hugest load ever onto my chest. Right after this she started moaning and her toes curled and her body cringed, she also even squirted quite a bit. She told me that this was the best feeling she ever had. We just sat there for a while soaking in our cum.
We continued to do this every day and had even given each other hand jobs. She is the best sister ever!