This happened when I was sixteen and my brother was twelve.
One summer we went on a family vacation. My parents rented a cabin on an island in Northern Minnesota. My brother and I shared a sleeping area that included a double bed and a small loft with a cot. My brother decided to sleep in the loft.
One night, after being on the island for about a week and with no privacy, I was so horny that I decided to chance it and quietly start masturbating. I stroked my clit, rubbing in little circles and pressing harder and harder as I got closer to coming. Soon I realised that the cot up in the loft was squeaking in a way that betrayed my brother up there wacking off too. I listened more closely and heard him breathing hard and softly moaning. Well that made me moan out loud too! My fingers were now sliding in and out of my drenched pussy making wonderful squishy noises. I couldn't hold off, when I heard him stifle a loud moan and heard the bed stop creaking suddenly I lost it and had one of the strongest orgasms of my life. We never talked about it and it never happened again. Too bad!