Ok, I just found that familias is the genitive form of familia in archaic latin, after being perplexed by its declension.
I used "the man of the house" as the closest equivalent to English and was marked incorrect.
Paterfamilias is not just a father but the eldest person in the household. It can be either father or grandfather.
Good q. The past is a foreign country though... the Familias might not have been a nuclear family, but a broader term to include extended relatives from a particular family. I kinda see Don Corleone for the movie The Godfather as a paterfamilias....