The correct answer to this open question is the following.
It seems that the way you pose your question is a little confusing.
What you probably want to know is the correct translation in Spanish of the sentence "The family is buying a brand new house."
The correct translations is "La famila está comprando una casa nueva."
When you are using the term "brand new," in Spanish you do not need to add an extra adjective for the word "brand." You just use "new-nuevo" to make it clear that the product -in this case, the house- is brand new.