OK. The reason I offered up "familia" is because the phrase in which it was used was "The U.S. stops somewhere short of the power over his familio" ... so I paid no attention to Pater Familius (which needs a genetive ending) and focused on the other phrase which, I would assume, requires an ablative (because of the "over"), hence "familia".
But now that this discussion has gotten going, I can't help but think back to the first time I saw "Life of Brian" and fell off my seat laughing at the skit on "Romans Go Home!!!" ....Romani eunt domus???? So, I feel pretty silly. But for all you folks that have never had the displeasure of studying a dead inflected language, the whole game is to obsess on the endings you tack on to words which can quickly turn a sentence like "The girl ate the chicken." into "The chicken ate the girl." (Word order doesn't matter nearly as much as in English.)
Forgive us for we can't help it.
Joanna