I've never told anyone this
When I was 14 we went on a family camping holiday to France. I shared a tent with my brother, who was 15.
We were not a touchy feely family at all and although we were quite close, we had never talked about intimate stuff. On the second night of the holiday, we did. We talked about people we fancied at school, progressing to how far we had gone. In my case, kissing and having my boobs felt and sucked, in his, kissing only. He asked if I had touched a boy's cock and I said only through his jeans and not for long. We talked about all of this stuff for about an hour, then fell asleep.
I am pretty sure he masturbated because I remember vaguely hearing that kind of movement through my half-sleep, and I was certainly wet from the conversation, but we didn't talk about it.
The next night was different. For one thing it was cold and we huddled together in his sleeping bag for warmth. We were giggling a bit and I suddenly noticed that he had an erection, it was hard not to as it was pressing into my belly! I didn't pretend to notice but I did start another conversation about sex and asked him if he masturbated, which he admitted he did. I said I did too, especially in bed at night and his cock was by now feeling very rigid, he was almost pressing it into my tummy.
As we started to drift off to sleep, he had his hands inside my nightshirt for warmth and he let them glide around to the sides of my boobs. I responded by stroking his back, then his belly. Now he moved his hands on to my tits! I nearly had a heart attack but didn't stop him, I could feel my nipples stiffening under his touch. As he felt and squeezed them, my knickers were getting soaked and his cock was straining at his boxers. I let my hand go to it, giving it a squeeze through his shorts and he let out a low moan. He kept one hand on my boobs and slipped the other one into my knickers. It was the first time anyone else had touched my pussy.
I moved my hand inside his shorts, feeling his stiff cock and squeezing it. I knew enough about what to do and just started slowly wanking his throbbing cock. He had his finger up inside me, fingering me, which was nice, but I had to move him on to my clit, my hand guiding him to the right spot. I rolled on to my back as he gained in confidence, rubbing my clit as I tossed him off.
His breathing quickly went ragged and he started thrusting really fast, so I automatically sped up and so did his hand. With a low moan, he ejaculated in my hand and stopped touching me for a few seconds, I just said 'don't stop' so he started to rub my clit really fast and I came too, thrusting my hips up against his hand and moaning so loud he put a hand over my mouth!
It was so so exciting at the time, though I have always been a bit ashamed of it since and have never told anybody else about it. We never did anything else but there are times when I still masturbate over it, I guess because the excitement at the time was way off the scale.
They tell me this kinda thing is common but I have never spoken to anyone else who has done anything with a sibling. Maybe, like me, they are too embarrassed to talk about it but still secretly wanking over it!
And I guess that is true of lots of the things that we all masturbate about, you wouldn't even want your partner to know some of them, real or imagined.