"Catching sis with her finger up her dress"
I was 11 then and my sister was 16, back then I didn't know anthing about sex or masturbation.
One night my parents were out at a business meeting when I heard moaning coming from the bathroom. I opened the door a tiny crack and what I saw amazed. There was sis with her hand up her skirt. At first I didn't know what she was doing then she turned around and I had a clear view. I could see her index finger right up her wet, dripping cunt. I suddenly started to get my first boner. I unzipped my jeans and touched my erection. It felt really good, so I started to do what I know now as jacking off. About three minutes later my sister gave out a big cry and took her finger out of her cunt. Just after I came for the first time.
Now everytime sis fingered herself I would be right there jacking off. 4 years on and she has never noticed I have been spying on her.