We know Marilyn has lived with the Munsters since she was a baby. Why did her mom leave her there? Who was her mom? We know it was Lily's sister, as Herman has said, "She's your sister's kid" to Lily. Marilyn seems to have no desire to track down her mother, and is content with living with Herman and Lily. Lily and Grandpa also don't seem to care where their respective sister and daughter is, or they do know and do not wish to discuss it. In Here Come the Munsters, she was Elsa Hyde, the sister of Herman. As that does not conform with the original plot, it is difficult to include it here.