3 Is not this the carpenter, the son of Mary, the brother of James, and Joses, and of Juda, and Simon? and are not his sisters here with us? And they were offended at him.
Jesus is identified with the same attributes by which a famous Messianist was identified (by the same kew-words: 'Joseph', 'James', Judas', 'Simon'):
"And besides this, the sons of Judas of Galilee were now slain; I mean of that Judas who caused the people to revolt, when Cyrenius came to take an account of the estates of the Jews, as we have showed in a foregoing book. The names of those sons were James and Simon, whom Alexander commanded to be crucified…"
(Josephus, Jewish Antiquities, XX: 5.2)
...to make again the point that he is identified (and hoped in) as the davidic Messiah, by the Jews. But Jesus is not the davidic Messiah. And especially he is not the demiurge, the 'carpenter'.
So a 'James' (just as a 'Simon', or a 'Judas', or a 'Joseph') could be the 'brother of the Lord', if this Lord was the same Creator, the god of the Jews (the 'carpenter'), in opposition to another god.
From this point of view, Gal 1:19 could be interpolated by Marcion himself to describe the judaizing James as brother of the his god, the demiurge, and not of Jesus.