How do you know Mary M., was Jesus' half-sister? They were lovers and I really doubt very much that he would take his sister as his beloved. What gospel is this in? It's certainly not in Mary's. Mary and Martha of Bethany may have been related to him, but I doubt Mary M. was.
You assume DaVinci had some inside track, some direct knowledge of the days of Jesus. He was not a mystic, and I wouldn't give him credit for anything other than guessing about those things.