Who is Aunt and Uncle and Beautiful Moon from Snow Flower and the Secret Fan and what is their importance?

Lily's Aunt and Uncle Wu have one female child, Beautiful Moon. They have had no sons, and they live with Lily's father and family because that is the way a family like this is structured according to the rules of their culture. He works hard and is really the only man in the household who shows Lily much affection and humor. Aunt is a toothless, comical character who tries to make everyone happy and to keep the atmosphere light. She mentions more than once what a good sex life she and her husband have had.