In the family way
The part about the “brothers” is relatively easy to deal with: in the usage of that time and place, that term included close relatives who were not necessarily children of the same parents.
The seeming slight against the Blessed Mother seems more difficult to deal with.
"Who is my mother? Who are my brothers?" And stretching out his hand toward his disciples, he said, "Here are my mother and my brothers. Whoever does the will of my heavenly Father is my brother, and sister, and mother."
It not only disturbs our long-nurtured Marian devotion, it makes Jesus look like a rude child (“I don’t need my parents, I’ve got my friends”), and it seems to clash with the wonderful depiction of Mary in the Gospel of Luke.
But it is precisely in the Gospel of Luke that we find the key to understanding what our Lord is saying, most specifically in one of the things Elizabeth says upon her Visitation by Mary.
“Blessed are you who believed
that what was spoken to you by the Lord
would be fulfilled.”
And in what Mary says at the Annunciation.
“Be it done to me according to your word.”
Christ’s message in this Gospel passage is that a relationship with Him must be based on living faith in God. As we know and as Luke emphasizes, Mary is first and foremost a woman of faith, who accepts and lives out the will of God – in a way more profound than our imagination can bear.
If we accept our Lord and Savior Jesus Christ, receive his grace and live according to God’s will, we are His brothers and sisters, and because of her faith, the greatest of our sisters is Mary (whom our Lord on the cross also presented to us as our Mother).
The Blessed Virgin Mary is the Mother of Jesus, the Mother of God, the Mother of the Church, and our Sister in Faith.