Aunt Effie!
I loved when she would show up. The episode where she fell off her roof while shingling it and Mama has to bring her home and care for her was funny. "oatmeal, oatmeal!" lol
Watching reruns on MeTV had me wondering why she was one of the characters on both the NBC series and syndicated. Come to find out she was played by one of the writers of the show.
By the way, the early seasons made it vague whether she was Mama's aunt, or Mama's sister, or sister in law, but the episode shown today, Mama's Girls, Mama referred to her as sister in law and mentioned they were kids together, so I guess she was either Carl's sister or Mama's brother's wife (did Mama have a brother?).