O'Tink, I found a YouTube video from Mary six-years-old, with her cowboy hat and a guitar as big as she...singing LONG TALL TEXAN, and they were calling her 'Mean Mary' then...somehow I doubt that she is mean! Her rendition was really good too! I am guessing that 'mean' Mary is an inside joke of a loving family...her brother's name is Frank, surname James? Well Frank James was the brother of Jesse, and it's possible Frank for the brother is not even his given name, rather another inside family joke...
So now I will go to the bio you found...
Here is Mean Mary age six, but my laptop could not open her bio... %$!!#@^&*!!! ...someday soon, I MUST make it more of a priority to update my system!!!