Jesus is identified with the same attributes by which a famous Messianist was identified (by the same kew-words: 'Joseph', 'James', Judas', 'Simon'):
(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.