Yes, it is definitely hard to define, especially nowadays.
The definition of a family used to be: a married man and woman with children.
And now, that definition isn't that straightforward - you have people who aren't married but have children, and consider themselves to be a family. You have gay couples with/without children. You have families with no children at all. You have a married man and woman with children who may not consider themselves to be a family.
It is very difficult to properly define the word family.