Marilyn remains steadfastly oblivious to the idea there is anything bizarre or frightening about her Uncle Herman and Aunt Lily, her cousin Eddie, or Grandpa, and she has become quite angry with anyone who "insults" the Munsters by pointing out their monster nature. Naturally, she is convinced that she is the ugly member of the family because she doesn't resemble her revived corpse uncle or her vampire aunt.
Marilyn's parents were never named in the series. However, it was revealed in the series that Herman had been built by the famous Dr. Frankenstein (making him the younger brother of the Frankenstein monster) and soon after adopted by the Munster family, a family of ordinary humans happily involved with mad science and magic; therefore, Marilyn must be the offspring of one of Herman's adopted brothers.
Marilyn Munster shares with her extended family a supernatural invulnerability, happily making meals of substances that would kill any ordinary human being and taking no damage from things that would wound an ordinary person, such as electrocution, being blown up, and drinking furiously boiling liquids. Despite her very human appearance, it is quite possible that she is no more human than are the rest of her family.