The standard Catholic view is that James the Just is the same man as James the Less, one of the Twelve. But among non-Catholic New Testament scholars this view finds little, if any, support these days. Instead, they identify him with the James who is mentioned in Matt 13:59, Mark 6:3, and Gal. 1:19, where he is named as one of the “brothers” of Jesus, and also in 1 Cor. 15:7.
The Greek word
adelphos can mean full brother and it can also mean half-brother, stepbrother, or even (possibly) a more distant kinsman.
Martin Hengel writes in *Paul between Damascus and Antioch: *"Now [after Peter’s arrest and escape at Passover 43] the leadership of the earliest Jerusalem community passed to James the brother of the Lord and his body of elders, which is mentioned for the first time in Acts 11.30. The Lord’s brother was one of the earliest witnesses to the Resurrection (1 Cor. 15.7), but not one of the Twelve. Luke mentions him here for the first time (12.17), and in so doing suppresses the fact that he was the brother of the Lord.”
N.T. Wright ties the loose ends together very convincingly, I think, in his book
The Resurrection of the Son of God. Discussing 1 Cor. 15:7, “Then he appeared to James,” Wright dismisses out of hand, in a throwaway parenthesis, the alleged identification of James the Just with James the Less: “(this clearly refers to the brother of Jesus, not to either of the members of the Twelve who had that name).” He then goes on to state his conclusion in a single carefully worded sentence: “Since he had probably not been a disciple of Jesus during the latter’s public career, it is difficult to account for his centrality and unrivalled leadership unless he himself was known to have seen the risen Jesus.”
There is a 472-page book, no less, dealing with every aspect of the “brethren” question:
Jude and the Relatives of Jesus in the Early Church, by Richard Bauckham. This one I haven’t read yet, but it’s on my list:
amazon.com/Relatives-Jesus-Church-Academic-Paperback/dp/0567082970/ref=sr_1_1?s=books&ie=UTF8&qid=1440449667&sr=1-1&pebp=1440449672525&perid=171EJ4Z3FMW21GWE6H59