Mine are 8 and 10, and if I don't see what happened first hand in their squabbles, I never take sides. The only punishment, is whatever toy or item, if there is one, that they are fighting over, gets taken away from both of them. That's it. They both will scream their sister is "lying" and I know they are BOTH can push each other's buttons, and are BOTH capable of getting physical with each other when they are very angry, and also BOTH capable of trying to manipulate me to punish their sister. I've heard about a lot of pushing and hitting, etc. and I've certainly heard a lot of fake crying too.
What I do is give sympathy to whomever comes to me crying, in the form of a hug, and offer of a diversion (like why don't you go and play with your DS for awhile). Later when everyone cools down, I talk to them individually. I listen. And I give advice on what they can do next time for a more peaceful resolution. But ultimately, they have to learn to work things out together themselves.