In the Vulgate Translation, Judas Thaddeus is refered to as “Judas Jacobi” translated to Judas of James. Would Judas be a son or brother of James?
That entirely depends on who Judas Jacobi is.
I don’t believe we have any direct identification for the Apostle Jude’s father, so he could have been the son of a James.
However, Jude or Judas is also given as the name of one of the Lord’s “brothers,” who had another brother called James, so that could work too.
Whether Jude/Thaddeus the apostle and Jude the Lord’s brother are the same guy is a debated matter. Ditto with whether James the brother of the Lord was the apostle James the Less. The most common stance currently seems to be that those were all different guys. So whether Judas Jacobi was the son or brother of a James may depend on whether Judas Jacobi is the Apostle Jude or Jude the Lord’s brother.
Could you clarify what you mean. Is the expression “Judas Thaddeus” used anywhere in the Bible, Vulgate or not? I’m looking in a concordance for Thaddeus and I only see two examples:
Matthew 10:3 - “Lebbeus, whose surname was Thaddeus”
Mark 3:18 - “and Thaddeus”