How is Christ a Son of David?

I'm listening right now to EWTN and a caller is having trouble understanding how Jesus Christ is from the house of David. The answers are coming I think from Fr. Mitch Pacwa. The trouble stems from a misunderstanding of what constitutes sonship. She's worrying over the fact that Jesus of Nazareth is not biologically related to Joseph. Two answers were given: one, there is evidence from ancient but extra-Biblical sources that Mary also comes from the house of David; and two, that the sonship of Jesus by David is a legal reality. First of all, there are two sources of Mary's Davidic lineage: the Proto-Evangelium of James (~ 120 AD) and a tradition from the same century that a relative of Jesus, presumably from his mother's side, being arrested and brought to Rome on fears that he would try to revive the Davidic kingship as an heir of that house. The man, who was a farmer, was subsequently released. Of course, many would object to these sources for being extra-Biblical. The second answer should be even more satisfying. Joseph's sonship of Jesus was very legal in Jewish society. Legality of adoption is not problematic at all because we are sons and daughters of God in the same vein: we are legally adopted in having been born again in the waters of baptism. Do we ever question how real (or unreal) our relationship is with God as his children? I hope not!