“Mummy Diller” was a human woman who is now an undead mummy.
A shot of Mummy Diller materialized on the ghost-to-ghost television channel in 1952 while her daughter, the witch Phyllis. She was, by then, a ghastly-looking undead mummy, though she nevertheless remained, as her daughter put it, "a real fashion nut", tastefully choosing her lipstick and what wraps to wear. She had a married a monstrous bellringer described by Phyllis as "a real swinger".
Phyllis also referred to Madam Mim as her mother; assuming that Mim is indeed a last name (or at the very least not just a first name), this would suggest that Mummy Diller is Madam Mim's sister.
A third sibling can also be deduced in the form of Medusa, also an aunt of Phyllis's, as while both Mummy Diller and Mim could plausibly hail from a Greek-oriented ancient world, the same cannot be said for bellringing Daddy Diller.