His being Messiah would have nothing to do with it. He lived in a Jewish culture, and it would be very odd for Him to leave His mother with someone who wasn't related to her.
Not really. The jewish culture was about to be drastically changed. And the brotherhood of believers in Christ would become even more binding than that of blood relations.
Matthew 12:50 For whosoever shall do the will of my Father which is in heaven, the same is my brother, and sister, and mother.