This is what happened yesterday:

See, Ana and Evangeline have an exercise ball that they've recently rediscovered. And so they've been bouncing up and down the hall on it.

I was cooking, and I saw Evangeline go up and down the hall on the ball. Then I saw Ana... and heard Evangeline yelling:

Eva: Ana! I was using that!
Ana: No, you were done with it!
Eva: No I wasn't! I just put it down for a minute!
Ana: A LONG TIME!
Eva: I'll tell Connie!
Ana: No! I'll tell Connie, you're just throwing a FIT to get me in TROUBLE.
Eva: AAAAAAAH! I'll tell Connie YOU'RE the one who's lying, to get ME in trouble!
Me: Okay, now you're both in trouble.

And I had to confiscate the exercise ball entirely, thus uniting them again against me. *sigh* But as I told them, anything that would cause them to act in such an unsisterly way (and believe me, I don't care who was about to lie to me, the other one would have been almost as bad - tattling for no reason but to get her way and get her sister in trouble! How unkind!) is not worth having around. Period.

On the plus side, neither of them actually tattled to me. They just threatened to. We're making progress! Sorta.