My headcanon is that Mary Ann's parents were killed in a car crash when she was very young, and she went to live with her Uncle George (his brother was MA's father) and Aunt Martha on the farm, who raised her.
Yes, she did refer to her mother a few times, but perhaps she sometimes calls her aunt that.
It just makes sense with what little info we were given about her.
Community content is available under CC-BY-SA
unless otherwise noted.