It didn't really seem playful in my opinion. It seemed more like she got angry and worried because he read the notebook. For whatever reason, be it its contents or the fact she keeps such a book.

To be frank my first reaction was that her family has some memory disorder hereditary syndrome and she thus need to keep a detailed log to function normally. Maybe it's affecting her sister far more, making her something Tsukasa is embarrassed of. But, yeah, that's an exceedingly early theory after so little revealed.

But in any case, it seemed to me she dropped her facade and got ready to deal with Junichi appropriately.

Edit: Actually I had another theory as well: The Ayatsuji sisters switch roles sometimes and thus need detailed notes to keep each other updated. They did look so similar!