The verse in Mark 3:21 says when his family heard this, they went to take charge of him, for they said; He is out of his mind" -
This verse says family which could mean mother also but is does not say Jesus mother, it only says family. I believe this is references his brothers and sisters only and I will tell you why in a sec...
The next verse in Mark 3:33,34 where Jesus is asking "who are my mother and my brothers?" and says "whoever does God's will is my brother, sister and mother" But this does not in itself implicate Mary his mother as one who did not believe he was the Messiah. Here, he is merely showing that membership into God's spiritual family, is more important than our human families.
This thought is brought out by Mark 10:29-30 it says "I tell you the truth", Jesus replied. "no one who has left home or brothers or sisters or mother or father or children or fields for me or the gospel will fail to receive a hundred times as much in this present age (homes, brothers, sisters, mothers, children and fields - and with them persecutions) and in the age to come, eternal life.
The reason I say Mary believed Jesus was the Messiah is that she knew she had concieved thru God's Holy Spirit and the angel told her she would have God's son. And that she should call him Jesus and that he would rule over the earth, his kindgom being forever. See Luke 1:29-33.
And Elizabeth, John the Baptist mother also told her she was blessed by the LORD - Luke 1:39-44, and Mary because of this knowledge song a psalm or song to God known as Mary's song, here it is and it also proves she knew the significance of her Son Jesus.
46 And Mary said:
"My soul glorifies the Lord
47 and my spirit rejoices in God my Savior,
48 for he has been mindful
of the humble state of his servant.
From now on all generations will call me blessed,
49 for the Mighty One has done great things for me—
holy is his name.
50 His mercy extends to those who fear him,
from generation to generation.
51 He has performed mighty deeds with his arm;
he has scattered those who are proud in their inmost thoughts.
52 He has brought down rulers from their thrones
but has lifted up the humble.
53 He has filled the hungry with good things
but has sent the rich away empty.
54 He has helped his servant Israel,
remembering to be merciful
How could Mary forget all this and think Jesus was not the son of God?