I think immediate family is plenty for the EOI. Parents, brothers and sisters, children, partner/spouse, and grandparents is fine. You dont really need to go in to cousins, aunts and uncles.

I think the idea behind it is that if someone applies for family sponsorship in the future, if they are not on your list, they cant claim family sponsorship. Its also just a bit of background checking to make sure none of your family are terrorists or something, and that you are who you say you are.

Thats what I think anyway!