...but my mother and her two older sisters all have the first name Mary but none of them use that name.

I can just hear the conversations around the house if they had used their first names....

"Mary stop teasing Mary and Mary, pick up your things before company comes. Mary, help your sister, Mary, with her homework. Mary, stop chasing the dog! Now you girls behave yourselves. Mary sit there, and Mary sit there, and Mary, you over there, no, not you, Mary, I want Mary there. Don't you girls ever listen?"

It boggles the mind!