James and Jude (aka Judas) were two of four of the brothers of Jesus. The title 'Son of David' is a royal title, the scepter of Israel would not pass from Judah:
"The scepter shall not depart from Judah, Nor the ruler's staff from between his feet, Until Shiloh comes, And to him shall be the obedience of the peoples. (Gen. 49:10)
So the king was always to be a descendant of Judah, Judah's older brothers got mad one time and killed everyone in this town, Jacob didn't think making their descendants kings was such a hot idea. The Davidic line known in Hebrew as Malkhut Beit David "Kingdom of the House of David” (
Wikipedia), was Christ’s claim to the throne of David (Matt. 13:55; Mark 6:3). Speaking through the prophet Nathan God promised David a permanent dynasty:
Your house and your kingdom will stand before me permanently; your dynasty will be permanent. (2 Sam. 7:25)
When Pilate asked Jesus if he was the king he could have just explained all this to him, but he didn't. When his enemies called him a Nazarene and a Samaritan Jesus could have just showed them his pedigree as being from the House of David, he didn't do that either. But the church knew Jesus was from the House of David and rightful heir to the throne, James and Jude would have been seen as royalty as well, That why James presided over the Council of Jerusalem:
When they finished, James spoke up. “Brothers,” he said, “listen to me. (Acts 15:13)
Peter had already spoken, so why would they listen to James? He too was from the House of David and if Jesus was the true king, James would have been considered a leader based on the fact he was the next oldest son in that family.
Tradition has long held that Jude was one of the brothers of Jesus, which is most likely the case for a lot the same reasons Jude was regarded as a leader.