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

Aunt Agatha is the sister of Bertha. She is married to Jonathan and has a young daughter named Frannie. She agrees to take in her niece Una when Bertha is afraid that Ulysses will kill her. The family eventually moves from the Nantucket lighthouse to a lighthouse on the Great Lakes.