Luke is not telling a fairy story, but recording history.
He tells us first about a family summoned, describing the bureaucratic order of Augustus and its physical impact on families in Israel, and providing what might first appear to be incidental detail about Joseph and Mary. Behind it all we discern the heavenly design by which all things are brought to pass in accordance with God‚Äôs will.
With this in mind, we can consider a family line: the detail is not incidental. Luke is making plain that all of this fulfils God‚Äôs promises concerning the Son of David who will sit on the throne of David. This is a royal household indeed!
Finally, we see a family growing. Mary‚Äôs child is born - born in humility, poverty and obscurity. The Lord of heaven stoops down to earth, coming into the world to save his people from their sins.