As we perceive, so we remember, and so each of us imagines our own little reality. Over time some habits, quirks, games if you will, emerge as shared realities. And then there are sisters, who bond in a magical way. For me, this little story illustrates how that can work.
Important note: No children were harmed in any events even remotely related to inspiring this story. All parents involved understood quite well that imagination can be a more effective disciplinarian than violence.