Although Joan Durbeyfield is a minor character in the novel, how does Hardy use her as a tool to bring out larger themes of Tess of the D'Urbevilles?

Tess's mother, Joan, is a a superstitious, yet positive
and strong woman who just wants the best for her daughter. She simply wants Tess to
marry a nobleman and have a good life. Joan is supportive and loving towards Tess no
matter what misfortune befalls her daughter. Joan represents the older and more
experienced woman of the time period, but does not inform her daughter of the ways of
the world or the ways of men--probably because such things were simply not discussed
outwardly at the time. After Tess returns home the first time after being with Alec, and
pregnant, she asks her mother why she never told her of the ways of men.  Joan felt
badly about that, but she didn't think that she would have to since noblemen were
supposed to be responsible and kind according to their social standing. In this, Joan
herself is also as naive as her daughter.  Joan steps up to the plate later and advises
her daughter not to tell Angel about what happened with Alec when she was a girl. She
specifically writes to Tess and orders her not to reveal anything about the baby she had
and who died. Sadly, we will never know if that advice would have saved Tess's marriage
because Tess does reveal her past to Angel.  So larger themes brought out through Joan
would be motherly-love, motherly-commitment, a woman's typical life at that time in
England, and a woman's perspective on male/female and marital
relationships.