Like most siblings, the girls fight about a lot of little things. One of them does something to get on the other's nerves -- usually on purpose -- and the other one objects.
I will say that most of the time they work it out themselves. One of the things I say a lot is this: "You two are sisters. It is your job to take care of each other. There are a lot of mean people in the world. You have to be nice to each other." They have heard this a lot, so they do try to work out their problems.
If they cannot work it out, one of them says, "Fine. I'm telling Mom."
She starts to tell on her sister, but it's a really just a threat to get her sister to give in. Instead of telling me what the problem is, I hear this, "MOM (in an angry voice)." Then there is a pause. It's like the threat of getting her sister in trouble just hangs in the air while the other sister decides what to do.
I'll say, "What?" Usually at this point the other twin finds a way to avoid getting in trouble. The response, suddenly all full of sweetness is, "Mom, I love you."
Then the two of them go off together to play until the next incident. I have to admit that I do wonder what the problem was and what the offending sister did to get herself out of trouble. I'm sure whatever the offending sister did to stay out of trouble was just a different form of punishment like playing her sister's game or allowing her sister to play with a favorite babydoll.