She automatically thought it was her sister

Picture this scene: S and D are standing next to each other on the ball field with their backs facing me. Another child, who was off the field, is running towards them. That child throws a ball and unfortunately it hits poor D from the back.
As soon as the ball strikes D, with the quick reflex of a predator in the jungle, or a sister who's been damned too many times, D makes a fist and hits S really hard. S starts crying and hits her sister back. This may have happened for a few rounds until someone broke them up.

The mom of the girl who had hit D with the ball starts apologizing profusely for being the cause of the hurt and the fight between the two girls, but I don't think it's necessary, plus I'm too busy laughing. I really wish I had it on tape.

For some reason it was really funny to me. The other parents probably thought I was mean to laugh and out of a dozen or so parents only I and this other guy (who admitted that it wouldn't be funny if it was his own children) found it funny.