My father never had anything other than boys, and my mother always wanted a girl. Try as they might, they just had tons of boys. When I was 6 they adopted a girl of also 6. Everyone was pleased, and she was quickly included into the family by everyone and we all took an immediate shine to her. Especially me.
We started playing "doctor" at 9. This progressed to fooling around by our early teens, and into actual sex shortly thereafter. We're both over 30 now. We have sex whenever we see each other. We also like to pretend we are twins when we do have sex. We've both had our shares of girlfriends and boyfriends, but we always kept it up even while in those relationships. She's actually married now.
We still have sex about 2 times a month, more when the family gets together for holidays. I can't even imagine the bricks that would be sh*t if anyone ever found out. It's been close a few times, especially when we were younger, but nobody's ever caught on.