When I was fourteen.
This happened about seven months ago, shortly before my birthday. Mum and dad were at a dance for charity, leaving my sister and me at home. My sister Tracey is eighteen, and she called her boyfriend over.
We sat and watched TV for a while and about 10:00pm I went to bed. As I lay there I could hear giggling and crept out of my room to investigate. I got to the top of the stairs and could see into the living room and saw Tracey and her boyfriend kissing and he was playing with her tits through her blouse. Next thing he has her skirt up and he slipped his hand down her panties and started stroking her pussy.
As I watched this my cock started to stiffen and was soon solid. I pulled it out of the opening of my pyjamas and started wanking as I watched them. A few minutes later I felt myself about to come so I crept to the bathroom but was too late and came over my pyjama bottoms and had to try and clean myself up.
Just as I was coming out of the bathroom I heard the front door shut and Tracey coming up the stairs. She spotted me before I made it to my room and also spotted the wet patch on my pyjamas, she just said she would speak to me later.
It was a couple of nights later and dad was away at a union meeting and mum went to visit her sister. Tracey and I watched TV, I then went to my bedroom and had just got into my pyjamas when she came in. She said she and I had to talk. She asked if I had seen her and her boyfriend's heavy petting and I said yes, and was sorry for spying. She then asked if I had wanked at this and since she had seen my wet pyjamas I said yes.
She said not to worry she wouldn't tell but I had something to learn yet. She told me to stand still and stripped me naked. I tried to cover up a bit but she said relax and enjoy. So I took my hands away giving her free access to my cock which she started playing with and soon it was fully erect. She said not much of a little brother now.
She then stripped naked and Wow! what a sight, as I enjoyed looking at her lovely tits and erect nipples, and oh boy, her perfectly trimmed pussy. She lay me on the bed and gave me the best wank I had ever had up to that time. I came over her hand and my stomach and it dribbled down my shaft and over my balls. She then lay back with her legs spread and let me play with her pussy lips and told me to put my fingers inside and give her a wank. This I did and the next thing she was bucking and my fingers were soaking wet. We lay naked with each other till we heard the front door open and she slipped into her room.
We now do it with each other regularly, even although she probably does it and probably more with her boyfriend. I think my big sister is realy cool.