I remember one afternoon I had bunked off school and was in my room, panties around one ankle and trying to get myself off. I was almost there, but couldn't quite get over.
Then, right out of the blue and totally unexpectedly, my brother walked in on me. For all of five seconds he stood there, mouth open staring at me. I must have looked a sight, heels up by my ass, knees wide apart, fingers up my pussy, and then he just turned and walked out.
As soon as my pre-orgasmic brain internalised that I had just been caught jilling by my own brother, I came sooooo fucking hard! And not just once. Four really good orgasms one after the other!
No, I would not have fucked him. No I would not even have masturbated him, but him seeing me was one of the hottest things that has ever happened to me.