I'm not saying that Lily isn't a vampire.

She's both in a way.

Traditionally, vampires and werewolves weren't always enemies. In fact, they were not even that different from each other.

In Slavic folklore, for example, werewolves would become vampires after they died.

Then there's Grandpa Munster. He seems to be Count Dracula, so we can assume that he at least shares some abilities with THE Count Dracula. And Bram Stoker's Dracula can, among other things, turn into a wolf. In my book, this makes him part werewolf. (Francis Ford Coppola seems to agree with this. His version of Dracula is basically a vampire-werewolf-hybrid.)

Traditional vampires, and werewolves were also linked to witchcraft. And who is known to use magic? Grandpa Munster.

So my theory is that Lily Munster comes from a family of werewolves, and like traditional werewolves, they turn into vampires after their death.

That's how a vampire and a Frankenstein monster can have a werewolf son. Lily was born as a werewolf, and then died and returned as the vampire we all know and love. The same happened to her father, and the same will happen to Lily's werewolf son Eddie, and her werewolf brother Lester once they die.

And as a side-theory to the main one, I think that in the world of The Munsters, some people are immune to vampirism, and lycanthropy. That would explain why Marilyn is a regular human despite being a member of the Munster family.

If I had to guess, I'd say that Marilyn's mother, and grandmother were both immune too. And since Marilyn's grandmother was Grandpa Munster's wife, this could possibly even be a link to Herman Munster's origin story.

I mean Herman was created by Dr. Frankenstein, the original mad scientist. Grandpa Munster is also a mad scientist. And we know that Frankenstein's assistant Igor is a relative of the Munsters, who later became the caretaker of Grandpa's Transylvanian castle.

So it seems likely that Grandpa Munster played some role in Herman's creation.

Possibly because his wife was a mortal human who couldn't be turned into a vampire and/or werewolf. And what would an immortal mad scientist like Grandpa Munster do if he was married to a mortal woman he loves? Find an alternative way to make her immortal of course.

So he and Dr. Frankenstein put their heads together, trying to find a cure for death, and the result of this collaboration is Herman Munster.