God is the One who defines that family line, no human being gets to expand or shrink that circle. It’s His covenant, His promise, His people.
I see this as the key to a correct understanding of the Abrahamic Covenant.
It would clarify the meaning a lot more though if we were to say "Jesus is the one who defines".
Concerning the house of David it was prophesied: "He shall build an house for my name, and I will stablish the throne of his kingdom for ever" 2Samuel 7:13.
It is because of this prophesy that there was such great excitement when Jesus rode into Jerusalem, and such bitter disappointment when He was crucified so soon afterwards.
Both Mary and Joseph were born into the house of David, although the throne had been empty for many years by the time Jesus was born.
So the people were right in recognizing Jesus as the son of David and rightful king of Israel.
The people were also mistaken, as many Jews and Christians are to this day, in not recognizing the full extent of His authority and power, or the magnificent size of His true kingdom.
Revelations 11:15 "The kingdoms of this world are become the kingdoms of our Lord, and of his Christ; and he shall reign for ever and ever."
Here we have the fulfillment of 2Samuel 7:15, but notice that the borders of Jesus' kingdom has been expanded far beyond the borders of the land promised to Abraham.
Of course all the land from the river of Egypt to the river Euphrates is given to Jesus too, and like the rest of the world He will do with it what He judges to be right.
His "rod of iron" might come in useful for that.
Another area of Jesus' authority is that he is also the king of heaven (John 18:36).
If Jesus was to establish the throne of David on this earth, that would not fulfill 2Samuel 7:13, because "the day of the Lord will come as a thief in the night; in the which the heavens shall pass away with a great noise, and the elements shall melt with fervent heat, the earth also and the works that are therein shall be burned up (2Peter 3:10).
In other words, it is impossible for an earthly kingdom to last forever.
A literal 1000 years would also have an end.
Another area of Jesus' authority is that He is also the high priest.
Just as the Levites made ceremonial intersession for man to God with their sin offerings in the temple, Jesus, a priest forever after the order Melchizadek (a higher and order than that of Aaron) makes real intersession for the sins of His people, being Himself the temple (John 2:19).
Note that without the existence of a Temple it is impossible to keep the law of Moses, and that one of the consequences for breaking the law is: "ye shall be plucked from off the land whither thou goest to possess it." (Deuteronomy 28:63).
Notice also that for all who can repent and believe in Jesus Christ, there is not only a temple and a high priest, but the entire "Greater Israel" and the blessings of the Abrahamic covenant are theirs too.