Some old traditions say that if a person is a werewolf while alive they become a vampire after they die. Maybe it's a progressive thing, like a caterpillar becoming a moth; Eddie will develop a more liquid diet when he becomes an adult.
If werewolfism and vampirism are just conditions a human can "catch" in this universe, it all starts to make more sense. The common thread, in any case, is they're all some kind of part-human. Alternately: Herman's testicles came from the corpse of a werewolf. Or Eddie is a vampire/dhampir with lycanthropy, I mean look at his ears and widow's peak. Does anyone remember which side of the family Eddie's werewolf uncle was on?
The werewolf uncle Lester was Lily's brother. It's also established that Grandpa has the ability to transform into a wolf, like Dracula. It's entirely possible that the two conditions go hand in hand, or else Lily's mother was a traditional werewolf and Lily just happens to take more after her father.
The real question is, if Herman was "made from spare parts," how can he have an identical twin?
The body that Herman's head came from had an identical twin. The "spare parts" were more or less identical as a necessity of the Frankensteining process: they had to be larger than average in order to be viable for the new body and more or less the same size in order to all fit together.
Actually, there were three identical monsters: Herman, Charlie, and Johann.
Four if you count Herman's younger twin brother Frank introduced in The Munsters Today.
Why does Marilyn Munster have the last name of her mother's sister's husband instead of her father, whoever he was? Did Herman and Lily adopt her? What happened to her parents?
Or, her father could have come from another family with the last name "Munster." It happens.
Granted that Marylin's suitors would be scared of the initial sight of her aunt and uncle, but doubly afraid that she'd turn into one of them.