[Dominic]

I think Mesmark has a good point.
Japanese people have, in the past at least, tended to have an extended family living in one house and therefore have a different concept of family.

When Japanese ask "How many people in your family?" They really mean "How many family members live in your house?" Otherwise the answer to that question could take ridiculous proportions, e.g. there are thousands of people in my family depending on how distant you want to go from the family center. And we all live in the human family. The whole world is my family.
In Australia, we don't ask "How many people in your family?" but "How many brothers and sisters do you have?" We assume you have a mother and father, and then ask what does your mother/father do? or ask for whatever specific information. We also assume you don't have a grandpa or grandma living with you, so it's not really part of the question.

Therefore, I think that Japanese and western people have entirely different communicative objectives, but trying to use one sentence structure to satisfy both.