We know that Jesus had half brothers.  In Mark 6:3-4 we read, "Isn't this the carpenter, the son of Mary, the brother of James and Joses and of Juda (Jude), and Simon?  And aren't his sisters here with us? And they were offended at him.  But Jesus said to them, A prophet is not without honor, but in his own country, and among his own kin, and in his own house."

And in Gal. 1:18-19 Mark tells us that "Then after three years I (Mark) went up to Jerusalem to see Peter and abode with him fifteen days, But other of the apostles saw I none, save James the Lord's brother."

According to scholars, Jude was most probably the brother of Jesus (half-brother--they had different fathers!!).  Maybe Jude didn't feel worthy to identify himself as the brother of Jesus since Jude had most probably not believed in Jesus divinity at first.

John 7-5  "For neither did his brethren believe in him."  So Jude identified himself as the brother of James.  And of more importance, as the servant of Jesus Christ.  In Acts 1:13-14, we find a list of those who were in the upper room waiting on the Holy Spirit. It says, "…and Judas the brother of James."  This is commonly accepted to be Jude.  He had accepted Jesus as the Christ.

Perhaps because of his reluctance to at first accept Jesus as the Christ, Jude is almost driven to proclaim the truth, and to be sure the people he wrote to knew when they were being bamboozled by false prophets.  He wanted them to recognize a traveling salesman who was selling snake oil.  Someone who was trying to deceive them and pull them away from the simple gospel.