So I was a 16 year old male with an average build, with a 13 year old cute blond haired sister who I usually get along with.
We have seen each other half naked quickly before but never in a sexual way, just changing near each other. But today was different. Now since I am a hormonal teen guy and I live with a cute sister, I am thinking about what she would look like naked and stuff.
I had my morning shower like usual and wrapped myself in a towel before walking back to my room. I was feeling kinda horny and daring, so since our parents were out for the day I thought that I could 'accidentally' drop my towel as I was walking past her room. I walked by her room and she was looking right at me as I dropped my towel and said sorry, by this time I had a huge boner but didn't care that she saw it.
I went to pick up my towel pretending to be embarrassed just as I heard her say 'It's ok, come in'. At this time I was very excited but still pretended to be nervous and embarrassed. I sat down on her bed with the towel over by legs. My sister asked me what masturbating was because her friend asked if she did it. I told her it is pleasuring yourself with your hand and that guys do it by pulling on their dick and girls rub their vag.
She asked if she could see my boner as she had never seen one before and without hesitation I lifted off my towel showing her my 5-inch hard cock with a big bush of pubes around it. I told her to watch and showed her how guys do it, but asked if she could take her clothes off too.
She quickly said yes and pulled off her shirt revealing her small but developing breasts turning me on way more. She then pulled down her pajama pants and undies showing me her wonderful young wet vagina. By this time I could cum instantly but held off and asked if she wanted to touch it. She soon learned how to jack me off and was doing it great, then I felt amazing and told her to keep going and I came and shot the hugest load ever onto me and her.
After this I started rubbing her, making me hard again and she gave me another hand job.
This continued whenever we got free time alone with parents not home. We would watch each other masturbate and help each other out giving us great memories and bonding moments.