I don't know if it's mentioned in the gospels*. It would be easy to google. It is mentioned in
Josephus's account of the stoning of James in Antiquities of the Jews Book 20 Chapter 9:1
"Festus was now dead, and Albinus was but upon the road, so he assembled the Sanhedrin of judges and brought before them the brother of Jesus, who was called Christ (Messiah), whose name was James, and some others [or some of his companions] and when he had formed an accusation against them as breakers of the law, he delivered them to be stoned"
I know amongst African tribes the terms father, mother, brother and so on are not a reliable guide to actual extended family structure. A tribal African will refer to any old man as Father (Babba) as a term of respect and to most elderly female relatives, aunts and so on, as Mother. Cousins would often be called brothers and sisters.
I do not know. Ehrman seems to be the go-to guy about these things at the present time. He makes the statement in a blog article
Gospel Evidence that Jesus Existed | The Bart Ehrman Blog
*EDIT: AI overview:
Yes, James is referred to as the brother of Jesus in the New Testament:
- Mark 6:3 and Matthew 13:55–56: James is named at the head of four brothers of Jesus.
- Galatians 1:19: The apostle Paul refers to meeting James, "the Lord's brother".