A tactic I've used to stop my older ones from hurting the little ones is to give them the 'job' of looking after them if I leave the room. Ask her if she'd look after the baby for a minute and thank her for being such a big girl and doing such a good job of it when you get back (if she hasn't smacked him off course. Start with leaving for literally a minute so you give her a good shot at being successful and build it up over time as she gets used to enjoying the praise. You might just have to take the baby with you for a while until she gets the idea.

Children have a real reason for what they do. Try to prevent leaving her alone with him, and find out what drives her. Help is in the book, Raising Our Children, Raising Ourselves. I learned so much about understand and responding to unwanted behavior from that book. You will find much help for many problems.