When I was younger, my family moved into a new house, and for a couple of months when it was being built we had to stay in an apartment. It was nothing fancy since we weren't going to be there very long, and I had to share a bedroom with my 17 year old sister, three years older than I was. Because of this I never had any privacy, and it was a problem since I was going through a changing period at the time. Any time I needed to masturbate it was either while going to the bathroom or in the shower, but being used to my own bedroom, it just wasn't the same.
One night I was lying in bed and I needed to cum like crazy, and I was constantly tossing and turning. My sister heard me and asked me 'what the hell is the matter with you, go to sleep.' 'I can't, my kitty is wet and itchy.' (That's what I was still calling it at that point in my life.) 'Then just play with yourself.' 'Now? With you here?' 'I don't care, as long as I can get some sleep.' I couldn't believe she said it, but it was pretty dark in the room so I kicked off the sheets and pulled my pajama pants and undies down, sighing when the cool air hit my wet slit. I started rubbing myself, and it felt so, so good. I hadn't masturbated in a couple days so that made it that much better, also because I was doing it with my sister right there. I felt my orgasm starting to build up, and my bare feet were kicking and sliding around the bed. I took a deep breath when I knew it was coming, and when I came I felt warm and tingly all over. It took a couple minutes before I relaxed and got up to wipe myself off with a sock. I flopped back on my bed and she asked me 'feel better now?' 'Oh yeah, so much better.' She rolled over and said 'good, now go to sleep.' I pulled my pants back on and drifted off feeling fantastic.
That was the only time I masturbated in front of her, since we moved into our new house not long afterwards and had our own bedrooms and bathrooms.