Who is Sally Davenant from A Test of Wills and what is their importance?

A cousin to Mark Wilton and widow of Hugh Davenant, Sally Davenant iks a member of one of the landowning families in Upper Streetham. She holds her opinions and emotions close under a veneer of polite chatter. She may be in love with her cousin, which is revealed when Rutledge finds out she was a nurse during the war in order to be close to Mark when the news of his death came.