Mary and Joseph’s First Born Son
In the fraternal organization, The Sons of the American Revolution, a member must be able to trace back his lineage through verified documents like a birth certificate to obtain membership into the organization. The stress of this particular organization is that each member must be related in the family tree by blood relation, which would exclude adoptive members of the family.
Right of the First-born Son
15 “If a man has two wives, the one loved and the other disliked, and they have borne him children, both the loved and the disliked, and if the first-born son is hers that is disliked, 16 then on the day when he assigns his possessions as an inheritance to his sons, he may not treat the son of the loved as the first-born in preference to the son of the disliked, who is the first-born, 17 but he shall acknowledge the first-born, the son of the disliked, by giving him a double portion of all that he has, for he is the first issue of his strength; the right of the first-born is his. 
In first-century Judaism this isn’t the case, after Joseph learns of Mary’s pregnancy, so long as he claims the child, even in an adoptive process, that child in Judaism becomes a blood relative in that particular family. There’s also another stress by Luke in his Gospel and that is Mary is betrothed still to Joseph, reminding us of her virginity, and that Jesus is Mary’s firstborn son. So what is the sum of the equation being a blood relative of Joseph and Mary’s firstborn son? Dr. Sri explains, “in this Davidic context, therefore, Jesus, as the firstborn son, would be seen as inheriting Joseph’s most valuable possession: his royal Davidic lineage…Adoption was covenantal, forming real family bonds between the two. The adopted child would be considered a true son and thus heir.“
 RSV Dt 21:15–17.
 Sri, 73.