How important is family to you? It is of course not all that clear what that means, mainly because 'family' can mean a lot of things. For instance, many people call those who are important to them 'family'. That renders the question redundant, obviously.

(A subordinate clause including the word "because" must be connected to a preceding or following main clause.)

The way you used "call" is not correct. The entity being named must go first. Here's an example of the right formula: [An Esperantist calls a tree "arbo", and a German calls it "Baum."]

Isn't the entity being named "family"? It's 'one defines family as those who are important to them, not 'one defines those who are important to them as family'.

Well, "define" and "call" are not the same. They take different templates.

I define 'family' as "those who are important to me."
I call "those who are important to me" (as) 'family.'

I define a 'door' as "a hinged device to allow ingress or egress."
I call "a hinged device to allow ingress or egress" a 'door.'

Alright, I see now after rolling my head around it for a while.