Who is Donna Caterina from Christ Stopped at Eboli and what is their importance?

Donna Caterina Magalone Cuscianna is Don Luigi's sister. She is about thirty, small, and stout. She is extremely strong-willed and fairly intelligent. She has many political connections in town and in some ways is at the heart of town politics, running the town. She quickly allies herself with Carlo, in part because she believes that Dr. Gibilisco's daughters tried to poison her so that they might have her husband. She does numerous favors for Carlo throughout the book, such as finding him a servant from the town.