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!

Ah, it wasn't quite that bad! Their first names were Mary but that was never used on its own. Instead they formed their names by joining Mary with their second name. My eldest aunt was called Mary Ellen in her formative years but, when she studied in France, everybody started calling her Marlene. It stuck. Same story for the other aunt and the old dear.