Less developed nations are to extended families as more developed nations are to

nuclear families.

matrilineal families.

families of procreation.

Less developed nations are to extended families as more developed nations are to nuclear families.

Less developed nations are the opposite of more developed nations. Following this analogy, we can conclude that extended families are opposite of nuclear families.

Extended families include your grandparents, ants, cousins, etc., whereas a nuclear family is just your and your siblings, with parents.