Oh, come on.
Actually it depends on whether the family in question is British or American.
If they're British you go through all the machinations above.
If the family is American, it's always singular. Same with nouns like "company" or "team". I have heard "team" used with plural verbs in the US recently, but I think there was a very strong association of the "team" being the plural members on it, not the entity, in that case.