You seem to be creating a distinction between James the Less and James the Righteous where most of us think none exists. Mark 6.3 lists the brothers of Jesus as James, Joseph, Jude, and Simon. Mary of Cleophas is listed as being mother of James the Less and Joseph. Acts 1 notes that Jude is the brother of James the Less. I suppose you maintain this is simply a coincidence, that James the Less had brothers Jude and Joseph, and James the Just did too.
However, the "brother" usage in the New Testament was written originally in GREEK, therefore using Old Testament Hebrew idiom to interpret the context of New Testament Greek is absurd.
On the contrary, the historical evidence is that St. Matthew wrote in Aramaic and translated into Greek, that St. Mark translated St. Peter's sermons for his Gospel (or perhaps translated his Gospel after writing the sermons down), and that St. Luke used both of them as a source.
Yeah, it's a disagreement. Eastern Orthodoxy also assigns separate Feast days to James the Less and James the Righteous, but opinion over there is also split as to whether this is a correct or incorrect division.