An experience to remember
This happened years ago when I was around 16. Both my parents work and post college my sister and I (three years older to me) are at home. Sis had joined some dance classes and as such I used to be home alone quite frequently and you might guess what used to be my favourite pastime. Watching movies and wanking off (I still do it quite often now and enjoy it).
What happened once was that when I was alone I got engrossed in watching a new porn flick I had borrowed from my friend on my comp in my room, and was slowly carressing myself. Especially the girl in the movie was a beauty and I imagined doing all sorts of things to her. So engrossed was I that I did not realise when my sis let herself in (her classes had been called off for some reason). I was watching the movie, slouching on my bed and was naked from the waist down. Since sis and I share the room, she had to come there and I did not realise when she entered. She obviously realised what I was up to and stopped by the door, for how long I am not sure. I was reaching my orgasm when by sheer chance I looked up and saw her. I was totally embarrassed and did not know what to do. To reach the bathroom I would have to cross her and I was too close. In fact I had already started cumming and as I hurriedly stood up to run I squirted, and in fact had let go of my dick in the process letting the cum fly all around, some of which landed on her feet.
To my utter surprise she started laughing and came close to me and said I knew you did it and she had wanted to see a man do it. She then proceeded to watch the porno flick with me and said that I should be careful, but wouldn't mind if I did it when she was there.
As you might guess I took up that offer and we watched a lot of flicks together after that. I have masturbated in front of her plenty of times and she too has joined in on occasions. In fact my sis was the first woman whose pussy I have seen as well as the first woman whom I have seen cumming.
We are both married now and live our seperate lives. We have never repeated this nor have talked about this, but for me it is an experience to remember.