I can't find much on these folks - seems they were yound when they started recording these:

Rebel Rousers - By Mean Mary & Frank James
Hard Times - By Mean Mary & Frank James
Something To Remember - By Mean Mary and Frank James

One music review said they are

"A duet comprised of a brother and a sister, Mean Mary ably handles the vocals and the two of them have exceptional technical skills with each of the variety of instruments they play."

In 1992 one CW re-enactor described Mary as "a 14 year-old Tennessean with a gravelly whiskey-voice."

I place Mean Mary" Civil War Music into Google, and found just a couple of appropriate articles.