Who is Aunt Agatha from Ahab's Wife, or, the Star-gazer: A Novel and what is their importance?

Aunt Agatha is Bertha's sister. She is married to Jonathan, with whom, she has a young daughter named Frannie. Agatha agrees to take in Una when Bertha is afraid for her safety. is afraid that Ulysses will kill her. The family eventually relocates to a lighthouse on the Great Lakes.