My sister and I have always been close. We played together a lot as kids, and as we got older we still remained fairly close. Anyways, one night, my sister asked me to hang out in this homemade 'tent' with her and watch movies. Out of the blue, she told me that the doctor told her she was beginning to develop.
This soon led to us discussing sex. My sister really didn't have much knowledge about sex, other than it causes babies to be made. She didn't even know how intercourse occurred.
So we began to talk about it and then my hormones took over. I asked her if she masturbated. She asked me what that meant, and so I explained that it was when you touch your 'privates' for pleasure.
She said that she sometimes would rub a pillow between her legs, but that was pretty much it for her. She said her friend, who we will call Alexis for the sake of the story, sometimes would get naked and touch herself.
By this point I was incredibly turned on, and so I asked her if she wanted to see me masturbate. She was curious, and agreed to watch. As I started, I explained about an orgasm and ejaculation. I asked my sister if she wanted to touch my penis, at first she was hesitant, but with some persuasion she was soon jacking me off. As I was about to come, I took control again, and shot my load into the carpet. Not much happened after that, but loads more happened between us the following night and several nights after, but thats it for now.