About two years ago we were on vacation in Florida with our parents. I was 16 at the time and my sis was 14. Our parents had their own room and my sis and I shared a room.
I think it was the second night of our vacation and I woke up in the middle of the night so horny I definitely needed to get off. I thought about getting up and going to the bathroom but I hate doing it standing up or sitting on the toilet. So I looked over and was pretty sure my sis was asleep and so I grabbed some tissues and hid them under the covers and started jerking off under the sheets. I was always looking over to make sure my sis wasn't looking or waking up. I guess as I got into it I stopped looking and out of nowhere I hear my sis say 'you couldn't tell but I've been watching you'. I was so embarrassed and turned over and acted like I didn't hear anything. Then she said 'you don't need to stop, I need to do it too'. So I turned over and asked her if she was serious and she said yea, and so we both started masturbating under the covers and listening to each other. It was such a turn on. Finally near the end I said that I would pull the covers down if she would too so we could see each other, and she said ok and we both pulled the sheets back and in about a minute we were both watching each other cum. It was so amazing.
Luckily our parents never found out. We would be at the hotel pool together and then when we went back to our rooms my sis and I would take off our bathing suits and get turned on looking at each other naked and then masturbate. Yea, kind of embarrassing doing it with your sis at first, and she said the same thing. But after the first couple of times we thought it was so hot.
For the rest of the trip we watched each other masturbate maybe five more times and now we still do it together once in while at home. My sis told me that one of her friends admitted that she masturbated with her brother once, but my sis didn't say anything about us. I think maybe more teens try it than admit it.