I have heard of any number of families where the children address Mom and Dad as "vous" and especially where the husband and wife use "vous" with each other. (There is a marvelous French film about one evening in the life of a very bourgeois couple like this.) The Chiracs were rumored to be such a family.

It tends to be an old fashioned thing these days - it changed as those of our generation were growing up (which is, I suspect, why my husband and his sisters just kind of avoided the whole issue however they could).

Then, too, there is the whole issue of the new daughter or son in law just entering the family - and to some extent, there is a ritual progression you just have to go through, especially if there is some "special circumstance" involved (like a second marriage, which is an issue in some families, or a foreigner).

I can recall all the "culture shock" of having the American management visiting our plant (in Germany), where we addressed everyone in the plant as Herr This or Frau That. The Americans were very adamant about using not just first names all around, but nicknames. David became Dave, and Robert was Bob - and the Germans (as most Europeans) were just as shocked as can be.
Cheers,
Bev