It's been long believed that James (the head of the Jerusalem church) was Jesus' biological brother. He's called "brother", for sure. However, the same Greek term for "brother" was repeatedly used whether it applied biologically or spiritually. In contrast, Catholic doctrine states Jesus had no biological siblings. (The reasons stem from an unbiblical doctrine, but that's not important here.) What is important is that a case for this belief can be made Biblically. Given the nebulous use of the term "brother", it's difficult to be dogmatic one way or the other. However, it's interesting to note that Jesus appointed John as the caretaker of His mother as He was dying. Why would He even had to have done that if He had biological siblings who could care for Mary, per Jewish culture? This seems to strongly suggest Jesus may have been an only child after all, and that James, the brother of Jesus, was His brother in the spiritual sense--in the same way I am His brother, an adopted son of Jesus' Father.