how to get along with your brother

I taught Frances to say to Clark, "Let's not fight." She likes to be in charge of things, you know, so she likes to be the one to make sweeping statements that get results. She's tried it several times now, and generally when she says it Clark turns, leaves the room, and slams the door behind him. But it does end the argument. Once last week when she said it Clark just said, "Ok," and that was that. I wonder when Clark's going to start saying it to her?