This happened a couple of years ago during the summer.I was probably around 16 and that summer was particularly hot. I had taken to walking around the house in just shorts without a shirt because I really hated the heat.
Well, one day I was sitting on the couch watching some t.v. when my sister who was 15 came into the room and started watching too. During a commercial she turned to me and gave me a purple nurple. She did it again a few minutes later.
To try and get her to stop I told her if she did it again I would give one to her. Now she had small breasts and I hadn't really thought of her in a sexual way before, but since girls breasts are private parts I figured that would get her to stop, but she did it again anyway.
She was wearing a tshirt at the time but I tried to grab her nipples. She didn't freak out though and she squeezed mine even harder.
We began wrestling on the floor and I decided to go under her tshirt to grab hers. As we were wrestling her shirt came up and she just took it off and continued her attack. It was here that I got an erection when I realized I was looking at my first pair of female breasts.
I was now trying to hide my erection while fondling her breasts while she continued to attack me. We continued doing this for awhile until we heard our parents coming.
I went into the bathroom and had one of my best masturbation sessions ever after that. I still think of it from time to time.
Nothing ever happened after that, but I wonder sometimes if she would ever do something like that again.