I was an average 16 year old guy who masturbated every day but I had never done anything with anyone else. I have a 15 year old sister who I get along fine with but I've never really thought of sexually and we have never really seen each other naked before.
This day was different though, I hadn't been able to jack off for about a week or so due to privacy and work and stuff so I'd been getting horny easily. My sister, Mary (I'll call her here), normally dresses a little skimpy with short skirts and stuff but not sluttily at all. Today right after I had my morning shower I came into the living room wearing only a pair of boxers as me and my family are fine with that and the others do the same sometimes.
My parents weren't home today so it was only me and my sis home. I came into the living room and sat on the couch across from the one my sister was on and we started watching TV like normal. But something caught my eye, she was wearing a fairly short skirt, not too revealing or anything if you aren't paying attention, but I happened to glance by her and see that she is not wearing any panties underneath, and I had a clear view of her pussy with a bit of pubes above. This made me very horny and very hard right away so I sat with my knees up to hide my boner, as I am wearing just boxers.
At this point I can't help but keep looking at her pusssy then I hear her say 'Like what you see?'. I was stunned and scared that she noticed, but I didn't care and said yes. In a sexy way she slowly brought her hand down and slowly rubbed her pussy. She stood up and came to sit beside me. I put my legs down so she could see my boner still in my boxers. Without hesitation she put her hand on my dick and played with it a bit then pulled down my boxers and jacked me off till I shot the biggest load ever onto the both of us. After, I rubbed her till she came.
Many incidents like this happened in years to come.