How did Niang break up the unity of her step children? (Chapter 9)

Eventually, the children attempt to get organized and revolt against Niang, by sending a letter to Father about how unequal their treatment is. Niang catches wind of this plot, and though she does nothing initially, she invites Big Sister to the second floor and converts her into a spy. The siblings cannot remain organized without the help of Big Sister and their attempts to change the situation cease.