Do we know who the descendants of Mary mother of Jesus were? So I am not asking about Mary and Joseph's genealogy. I am asking about their children's descendants.  So Jesus's siblings' children, their children's children, etc.  Whenever I search for this, I only receive results about their family history genealogy.  So essentially, what happened to Jesus's nieces and nephews?
Answered by my elder Rick Calvert from Hope Christian Church:

Cody,

The only historical genealogical information of Mary’s family after the birth of Christ are the names of her four other sons. 

Scripture states they were named: Joseph, James, Jude, and Simon. These were the half-brothers of Jesus. Half-brothers because Joseph was only the paternal father of Jesus, he was not the material father of Jesus, the Spirit of God was. 

The last three of Mary’s son’s mentioned are not to be confused with those who were disciples of Jesus by the same name. 

Here are some passages where the other sons of Mary by Joseph are mentioned (Mt. 12:46; 13:55; Mk. 6:3; John 2:12; 7:3, 5, 10; Acts 1:14; 1 Cor. 9:5; Gal. 1:19).

Also, keep in mind that all of the Jewish genealogies dating from Adam to Christ were burned and destroyed by the Romans in 70 A.D. with the destruction of Jerusalem and the Jewish Temple. The purpose of the genealogical records preserved in Scripture was to identify the Messiah when He came, which they fulfilled (Mt. 1:1-17; Lk. 3:23-38), thus, there was no reason for their preservation any longer and God let them be destroyed by the Romans in the late first century. Therefore, no person today can actually prove they are Jewish, nor can they prove their Hebrew tribal genealogy. This also implies that no one can trace their ancestry back to Mary, Joseph, or Jesus, most likely by God’s design.

Warm Regards,
Rick