When I read this, my first thought was to wonder why the boy was aggressive. Did he learn it from an adult? Was he protecting himself in some way? Does he have a problem with anger generally? The answers to such questions are important if a therapist is to understand what is going on with the little girl.
Yes, it is possible that she learned that the best defense is a good offense. But it’s also possible that the same things that were troubling her brother are troubling her.
Absolutely, children of this age can and do change behavior if the adults both understand the issues and provide some good role modeling and consistent supportive correction. Since this behavior is true of more than one sibling, I do suggest that the best thing to do is to see a family counselor who can help the parents understand what they need to do at home to help both children.
I wish you well.