This is a useful phrase. Let's say you want to say that you are the eldest amongst your sibling. Then we include your parents. You say:
I am the eldest in a family of six.
When you say that, is your family's total number is 6 or 7?
This is a useful phrase. Let's say you want to say that you are the eldest amongst your sibling. Then we include your parents. You say:
I am the eldest in a family of six.
When you say that, is your family's total number is 6 or 7?
You are included as you are talking about your part in that group.
So the total number of that family would be six.