Rule #1: Keep Out Of It
Rule #1: Never intervene on one side or the other unless there's a possibility of harm.
By harm I mean injury -- not minor pain. Let's say you're paying bills at the kitchen table when a high-pitched scream erupts from the next room. You stick your head in the doorway and find your 6-year-old son sitting on top of your 4-year-old daughter -- the source of the screaming -- and hitting her on the back. A typical reaction would be to single out your son and say, "Stop hitting your sister right this minute!"
In return you'll get: "But she messed up my cars and she was pinching me."
To which your daughter will retort: "I did not. He never lets me play with anything."
Followed by: "I do too, but you always mess up my stuff."
The statement "Stop hitting your sister!" adds nothing. Your son knows perfectly well that he's not supposed to hit. All it does is put you in the middle, a place you don't want to be because there's no escape.
To put Rule #1 into effect, you have to make a swift decision. Do you feel that your son could seriously hurt your daughter? If not, then the intervention is simple. Just say, "The two of you -- stop it, now."
The squabbling may continue -- often it will. If your son doesn't immediately comply and get off your daughter, simply repeat yourself -- "The two of you, stop it!" -- and lift him off. You'll have accomplished what's necessary: stopping the hitting without taking sides.
But let's say your daughter's picked up a solid metal fire truck and is about to give her brother a backhanded bash with it. Solid metal fire trucks can cause harm. They should not be used as weapons. It's for such situations that you want to reserve your sternest voice: "No. Put down the fire truck." Take it out of her hand if she doesn't let go of it right away.
Then say to her directly: "The fire truck could really hurt somebody. You cannot hit with it."
The message is basic: Harming others is never okay. There are no reasons that make it okay. It can never be allowed to happen.