Why does the Cave of Treasures give Mary a family tree?

The author says the old genealogies were lost. After the destruction of Jerusalem, he writes, no Hebrew, Greek or Syrian writer could show where the heads of families took their wives, and the books of the Jews had been burned three times. This left a gap that opponents used. The book says the Jews called Mary an adulteress and pressed the Church to show who her fathers were.

The answer is a table of sixty-three generations from Adam to Christ, written for a reader named Nemesius. It runs through David and Zerubbabel down to Matthan, whose two sons were Jacob and Yonakhir. Jacob was the father of Joseph, and Yonakhir, with his wife Dina or Hanna, was the father of Mary. Joseph was therefore the son of Mary's uncle, and she was given to him so that he could care for her. The author's point is that Mary herself, and not only Joseph, came from the house of David and of Abraham.