There had been an argument going on and Sachiko rushes out and runs into Yumi, knocking both of them to the floor. Yumi thinks Sachiko choose her to be her younger sister (petite souer) because she wanted to avoid playing Cinderella and having to dance with a man. Sachiko dislikes men. Intensely.
The "older sister" presents her rosary to her prospective "younger sister." It doesn't always go as expected, though, since Yumi turns Sachiko down. It looks a lot to her like Sachiko was just going to use her to get out of a play, and not that she cared about Yumi herself.
A bet is established that, if Sachiko can get Yumi to willingly become her petite souer, she can drop out of the role of Cinderella. One the other hand, if that happens then Yumi will be the one to play Cinderella.
