Here is the KJV translation followed by a commentary.
Mark 3:31 There came then his brethren and his mother, and, standing without, sent unto him, calling him.
32 And the multitude sat about him, and they said unto him, Behold, thy mother and thy brethren without seek for thee.
33 And he answered them, saying, Who is my mother, or my brethren?
34 And he looked round about on them which sat about him, and said, Behold my mother and my brethren!
35 For whosoever shall do the will of God, the same is my brother, and my sister, and mother.
You see here that Jesus considers the multitude that sat to be doing the will of God. How are they doing the will of God? They are waiting to learn Torah from the Master. It is God’s will that every man studies Torah. It is a rabbinical teaching that when two or more are gathered together to read Torah, God will be present.
Here Jesus makes a comparison between those who stand and those who sit. Those who sit are ready to study Torah, Jesus calls them brethren.
(Perhaps Jesus is suggesting that there is another type of mother - the gathered disciples? Mother M. Angelica, Foundress of EWTN says “The Church is a Mother because she is a Bride who is forever bringing forth children of light …” And from RC Catechism 169: “Salvation comes from God alone; but because we receive the life of faith through the Church, she is our mother: "We believe the Church as the mother of our new birth, and not in the Church as if she were the author of our salvation."55 Because she is our mother, she is also our teacher in the faith.”)
The Aramaic colloquialism to sit, means to be ready to learn, to be open minded, to sit at the Master’s feet, we might say today. The colloquialism to stand means to challenge the speaker. (In the parable of the good Samaritan, the disciples sat, and the lawyer who challenged Jesus, stood up.)
Here the Mother, and His brethren are standing, and calling. This shows that they are not ready to study under the Master. They are in fact challenging what He is doing - He should leave the disciples and come home with them.
Jesus had said "A prophet is not without honor except in his hometown and among his own relatives and in his own household." Mark 6:4
(Mothers, of course, don’t learn from a twelve year old child, they rather boss them around - as Mary did Jesus the adult, at the wedding in Cana. Siblings also don’t obey one another. Only much later during Jesus’s ministry did James became a disciple.)
We who read this passage are being reminded (by the multitude) that though we may have found Jesus we are, perhaps, just like his family, seeking Him while standing outside the synagogue, not yet ready to enter into it, to do the will of God. We only call His name. The Master warns us:
“Many will say to Me on that day, 'Lord, Lord, did we not prophesy in Your name, and in Your name cast out demons, and in Your name perform many miracles?'23"And then I will declare to them, 'I never knew you; Begone ...' Mt 7:23 a