When Anna was 17, she was something of a bitch really.
Always thought that she was oh, so much better than me, and she never once lost an opportunity for a put-down or to embarrass me.
One day, I guess I snapped. She made one snide comment too many - one nasty little dig. She had been swanning around in the kitchen. Our parents were out and we had slept in. I had treated myself to a full-on fried breakfast. (Something Anna hated - only because I liked it.) She fussed around with her organic muesli, and as she walked past the table, she quite deliberately knocked my orange juice into my breakfast.
With a smirk, she just turned her back. I was white-hot angry! I spun her around and slapped her hard across her selfish face. Then I shoved her-hard. She went over backwards and I went with her, slapping her, calling her all the names I could think of.
Somehow, in the middle of all this anger (she was livid that I dared to confront her) something changed. I don't know what it was, but I felt her pull me towards her by my waist and I felt her legs open under me. She was only in a robe and I was wearing nothing but a pair of boxer shorts.
I was hard - where THAT came from I don't know, and it was like my mind was detached. "Your cock is pressing against her cunt". Anna writhed and started to moan, I stopped trying to hit her and tugged her robe off her shoulders, exposing her small boobs. She seemed to like that, because she clutched me to her harder and I grabbed one of her breasts.
The next thing was a series of "Huhhhh Hunnnnnnh Huuuuuuuuuuuuuunhs" from her and somehow, I knew that she had cummed. I had never seen a girl orgasm before, but she did.
And so did I. By now, my cock had worked out of my boxers and was pressing hard against her panties and the top of her thigh.
I came hard all over my sister's crotch.
After? We just lay there, me on top of her, as our breathing calmed down a little. Then Anna kissed me, and it wasn't a brother/sister kiss. It was a full on snog. I was hard in an instant. She reached down and took my cock in one hand while with the other, she pulled her panties aside.
That was the beginning of three years of exploration with each other of every possible kind. It was stunning, debauched, disgusting, amazing, wonderful, erotic, perverted - pick any one you like.
The only thing was that our parents noticed a change. "You guys are getting on much better these days" remarked mom a few weeks later. Yep, we sure were, no matter which of us was getting on the other!