What really confused me was that her brother was only nine at the time. I had no idea what sex was about at age nine. I would appreciate an answer. I would like to be able to help her in some way, perhaps to refer her to some good counseling. I believe it is possible.
Some children mature faster than others.
Girls can have their periods as young as eight or nine, and theoretically are physically capeable of becoming pregnant at that age. We are taught through social beliefs that this is not acceptable, usually under all conditions, few are told it's okay if you keep it private and don't impose it on others. A woman I know was raped by her brother also. She told her mom and her mom didn't do anything about it. The girl grew up thinking boys could do whatever they wanted to girls and girls had no right to complain.