Missed part of this on my first time through, when I replied.
To my knowledge, Scripture says nothing about whether or not Mary did or did not have any brothers or sisters, or about any of her other family members outside of Elizabeth (and Zachariah) who were 'kin', John the Baptist, and Heli (Joseph "was ... of Heli", meaning in the context and lineage used in Luke, that Heli was the father of Mary, and Joseph is therefore the son-in-law of Heli. You might notice that "
the son of" is italicized, meaning that it was 'supplied' by the translators, but not found in the Greek text, here.), before the birth of Jesus. It is pure speculation to claim that Mary had no brothers. And whether or not she had brothers, is entirely immaterial to the 'blood lineage', of Jesus, as well.
1. (1-5) The request of Zelophehad’s daughters.
Then came the daughters of Zelophehad the son of Hepher, the son of Gilead, the son of Machir, the son of Manasseh, from the families of Manasseh the son of Joseph; and these were the names of his daughters: Mahlah, Noah, Hoglah, Milcah, and Tirzah. And they stood before Moses, before Eleazar the priest, and before the leaders and all the congregation, by the doorway of the tabernacle of meeting, saying: “Our father died in the wilderness; but he was not in the company of those who gathered together against the Lord, in company with Korah, but he died in his own sin; and he had no sons. Why should the name of our father be removed from among his family because he had no son? Give us a possession among our father’s brothers.” So Moses brought their case before the Lord.
a. Then came the daughters of Zelophehad: Normally, the land inheritance should be passed from a father to his sons in Israel, not to the daughters. What then in the case of Zelophehad’s daughters, whose father had no sons? Will their father’s inheritance simply be assumed by someone else, or will his name live on through his inheritance?
i. In generally, the system was not completely unfair to women. A woman received a dowry from her father as a wedding present. Typically, the father required his potential son-in-law to provide much if not all of the dowry. A dowry might consist of clothes, jewelry, money, furniture or more, and it was thought that the dowry could help provide for the woman if her husband left her or unexpectedly died.
b. So Moses brought their case before the Lord: Moses did what he should when faced with a new situation: He sought God.
2. (6-11) The settlement.
And the Lord spoke to Moses, saying: “The daughters of Zelophehad speak what is right; you shall surely give them a possession of inheritance among their father’s brothers, and cause the inheritance of their father to pass to them. And you shall speak to the children of Israel, saying: ‘If a man dies and has no son, then you shall cause his inheritance to pass to his daughter. If he has no daughter, then you shall give his inheritance to his brothers. If he has no brothers, then you shall give his inheritance to his father’s brothers. And if his father has no brothers, then you shall give his inheritance to the relative closest him in his family, and he shall possess it.’ “ And it shall be to the children of Israel a statute of judgment, just as the Lord commanded Moses.
Second, the phrase "David's throne". A red herring, as you here phrase this, unfortunately.
Actually Jesus, as the offspring and 'Son' of David, is to be 'invested' on David's throne. (I Ki. 9:5; Ps. 132:11; Isa. 16:5; Lk.1:32) However Scripture does not expressly declare when that happens (or happened), that I find, although the tense used is yet future as of Lk. 1:32. (But Matt. 2:2 has the 'Magoi" saying that he was born King of the Jews, And Jesus is referred to, in some way. by that title 16 other times in the gospels, including the inscription Pilate had nailed to the cross.) And in the same future wording, it is states in Lk. 1:33 that Jesus - well let's just see what some of that section of Scripture actually does say, when Gabriel visited the incredulous Mary. (My emphases.) Yet future, at that time. And certainly Jesus is not yet "reigning over the House of Jacob" in any expected way that I see, in the usual sense and meaning of these words. Yet he is and was King of the Jews. A contradiction, here? Absolutely not! Merely multiple facets given of His royal kingship, for He is also simultanerously the Son of David, Son of Abraham, Son of Adam, and Son of God. (Matt. 1:1, 20; 12:23; Lk. 3:23, 38) And any one facet does not automatically cancel out another facet, here, or anywhere else in Scripture, for that matter.
An example of what I mean: I happen to have some second, third, and/or fourth cousins that are variously married to other second, third, and/or fourth cousins, that I am aware of. (I actually have some double, triple, and quadruple cousins that I know, thanks to someone else who didn't have a life,

and took the time necessary to research all this.)
And in several instances, the parents of children are also cousins, and even double and triple cousins of their own offspring, in a couple of instances. The fact that they are the parents, does not mean they are also not still cousins. Granted, one is far more important than the other, but the 'lesser kin', is still entirely valid 'kin'. Think about it.
Ed