S.,
I have two 2.5 year olds and we had double the issue with hitting. We firmly believe in Positive Discipline. It makes no sense to us to say "Don't hit!" while slapping the child on the hand.
Anyway, the suggestions to redirect are good. However, your daughter is upset and doesn't have a constructive way to dispense of her anger. I agree that if your son has initiated the situation, there should be a time-out for his behavior. Then, tell your daughter that you understand that she's mad and needs to use her words. I have said to my children. "Tell Mommy you're mad. Say, 'I'm MAD, MAD, MAD!'" I shake my head back and forth when I say "MAD". I do this a few times and they either get it, or they crack up. Either way, the anger has been diffused.
Another option when there is just no reasoning is to grab a crayon, pen, whatever and a piece of paper. Tell your daughter, "You seem very angry. Show me how mad you are." Then, scribble on the paper to show her what you want her to do. She can express her anger on the paper instead of hitting.
If our children try to hit US, we catch their hand in mid-swing and say, "No, we touch Mommy/Daddy gently. We do not hit." At the same time, we are stroking their hand on our arm or leg to show them what gentle touching is.
It is DEFINITELY not an overnight fix. However, both of my children now think about it when they want to hit. If their arm goes up, you can tell that they hear the words to 'touch gently'.
Everyone has different parenting techniques, but I hope that one or more of these suggetions can help you.
~ L.