When I was 14 years old, I had one sister, who was one year younger than me, 13. My sister, my parents, and myself lived in a apartment that we had just moved into about a couple months ago. It was two bedroom, so my parents shared a room and my sister and I shared a room, but we were both on separate beds. I had a nice developing body and my sister's was coming along.
I knew that she masturbated because I saw her doing it at night once. I don't think that she knew that I knew. This one particular night I was very horny and I wanted to have some fun. It was about 11:00 at night, and we all went to bed at the same time. I laid on my back in my bed. After about five minutes of waiting, I slipped my pjs to my knees, and my top over my boobs. (I do not wear underwear with pjs.) I was still under the sheet, and I pulled the sheet to my stomach exposing my breast. I began to masturbate hoping that she would notice me. I did for a few minutes and I could tell in the little bit of moonlight that she was looking at me. The I got very exited and hornier. My pussy was soaking wet and I'm sure hers was too. Then I got a little braver and I completely moved the sheet off letting my wet pussy be in the cool air. I kept rubbing and I felt 'it' coming.
I made it more dramatic than it needed to bed. I breathed hard and made moaning noises (not loud enough for my parents to hear) and I squirmed a lot. The I calmed down, covered back up and pretended to go to sleep. After about five minutes I could tell that my sister was masturbating too. This is exactly what I wanted to happen! She stayed under the sheet and in her pjs, but it was thrilling to watch my younger sister masturbate. And it made me horny again thinking that she was masturbating because of me. Eventually I could tell that she was having her orgasm. Squirming, and the usual stuff. She stood up to go to the bathroom and I could see that she had a little wet spot in her pants.
I loved the first experience I had with this. I did it almost every night and she would follow after me. We never 'interrupted' each other while doing it, but I still loved it. We did this almost every night until I was 16 and we moved to a different house and had our own room. But still, It was amazing watching her without her knowing!