Alright, I'm setting here watching the Texas Chainsaw Massacre, and I know that ol' Leatherface is based on the crimes of Ed Gein, and even the house is loosely based on his home. Ed Gein however lived all alone and was somewhat of a recluse. I also know of that old Scottish clan that was reputedly murderous cannibals, long, long ago. So far as I know though their have been no families of serial murderers in American history, but would welcome reports, theories or legends otherwise.
I am also familiar with the Starkweather murder spree and the Copeland murders and robery, though these were couples so I wouldn't consider them murderous families.