Answer:
The family lives in a big city in a flat in Wales. <<< Answer
Tell me, what makes the most sense to you?
If you were to say...
The family lives in a flat in a big city in Wales.
If you were to say...
In Wales the family lives in a big city in a flat, it doesn't connect properly.
Which one sounds proper to you? :)
We turned the sentence into an Independent Clause by adding the following so that it can stand alone as a sentence.
The family is a subject that has a verb getting attached to it which in our instance is "live".
In wales is one of two prepositional phrases as it answers the wherein our grammar structure. EG: In a flat is the other prep phrase.