Was it ever explained how Frankenstein and a vampire had a wolf for a son. Was Eddie adopted?! How in the heck is Marilyn related to any of them???

Someone on Lily's side of the family was a wolf. That's why Eddie is a wolf-vampire combination.

Lily has a sister who gave birth to Marilyn. For some reason, Marilyn didn't turn out like they wanted her too.

Now if we can only explain how the creature from the black lagoon is an uncle. :)

Having the Creature for an Uncle does sound a little fishy...