I was 15 when my 17 year old sister caught me one mourning in bed playing with myself under the sheets. She picked the lock, walked in and ask me if I was jerking off. Said nothing, tried to hide it but she walked over and yanked the sheet off. She just stood by the edge of the bed, looking at me laying there naked with a hard on and a piece of toilette tissue by my side. "Oh wait till I tell my Mom about this" while telling me to show it to her. "Come on let's have a look at it" chasing me around the bed, trying to pull my hands off my dick and telling me to do it in front of her. Horrified about her telling my parents, I had to do it just to keep her quiet. I remember her closing the door, telling me Mom and Dad weren't home and sitting in front of me watching stroking my dick. "Do you think about naked girls when your do it" she asked me while cupping my balls in her hand and telling me to hurry up and cum. I was so embarrassed I wasn't getting anywhere so she took me inside the bathroom and did it herself. "I think you look better naked than dressed" as she stroked me all the way to an orgasm. It was embarrassing but at least I didn't have to worry about her telling my parents. As expected the whole thing turned into mutual masturbation every time our parents left home. Only thing that made her stop was having a boyfriend. We never speak about it but with both know what we did. I guess we both have reasons to keep it secret.