How many people are in your family?
is really, in standard English:
How many people are there in your family?
There are six people in my family.
Proof of this fact is that, the answer to the question: How many people are in your family? cannot be: Six people are in my family. That is not idiomatic because a native speaker just wouldn't say that except in some limited circumstances, but not in general when discussing how many members a family has.
Compare this question about the number of family members to a question about where people are:
How many people are in the garden? Ten people are in the garden.
"in the garden" is a location. For the "family" question ("How many people are in your family?", family is not a location. That's why it sounds funny to say: "Ten people are in my family." and it sounds natural to say: "There are ten people in my family".
How many people is a noun phrase.
Alternative way to pose the question: How many family members do you have?
This pattern uses: There are, There is, and the question forms.
There is and There are are used as existential clauses to show the existence of something:
There in "There is" and "There are" is called a dummy subject. Another example: it. It is an interesting movie. "it" is a dummy subject.