I have no idea, but I just wanted to sympathize. My DD6 is just like this. Very BIG emotions. With her, everything is dramatic.
But I disagree with the poster who said to make the sister share the paper. If the notebook is the sisters, then I don't believe she should be forced to share. She should be talked to and encouraged to share....and I talk to her about the Golden Rule, how Jesus would want us to share, etc, but I don't make them share their personal belongings.