How does the authors characterization of Bertha help reveal Delia's attitude toward Syke's affairs?

Bertha's place in Syke's life, and Delia's indifference to her presence, serve to illustrate her apathy toward her marriage. Delia's focus is not her marriage, and she no longer loves her husband. She merely wishes to hold onto the one thing that's hers.... her home. Based on Bertha's description, she could be anyone, and if it wasn't Bertha, it would simply be another woman.