Find key. Does she do this with others? At school? If she can control herself in other contexts then it is just carrot and stick (not literal) make chart, every day there is peace they both get a dollar (or something) every time she beats him you give him one of her dollars. Don't get mad, don't lecture. Model self control and wait for the teachable moment.
Child rearing. It seems that you have lost control on your children and need to assert more authority before your girl gets too big to be disciplined. You need to find out the reason why and set up some boundaries. Perhaps she needs to see a psychologist or her younger brother is egging her on. Too many possibilities. In the meanwhile, do tell that some day soon , her brother will likely become bigger.