What is the basis of the argument between Coomy and Roxana at the beginning of Chapter 9? How is this argument different from others they have had?

Coomy and Roxana argue about the cost of caring for Nariman and Coomy thinks Roxana and Yezad are accusing her of stealing from her father. Basically, the argument is no different from any other. They are always arguing about money and the care of Nariman.