Matthew 13:54 And when he was come into his own country, he taught them in their synagogue, insomuch that they were astonished, and said, Whence hath this man this wisdom, and these mighty works?
55 Is not this the carpenter's son? is not his mother called Mary? and his brethren, James, and Joses, and Simon, and Judas?
56 And his sisters, are they not all with us? Whence then hath this man all these things?
57 And they were offended in him. But Jesus said unto them, A prophet is not without honour, save in his own country, and in his own house.
58 And he did not many mighty works there because of their unbelief.
So? Mary was his mother. He had a group of relatives there, that many of us, including people from early days of the church believe were the children of Joseph and a first wife. This is the early belief of the church, btw. It is still the major belief of the Orthodox church, I believe.
If Mary had had other children, Jesus would not have given her into John's keeping.
If Jesus had had younger brothers and sisters, they wouldn't have dared try to come up and take him away because they were worried about his mental health. One did NOT treat the elder brother of a family that way. But one certainly wouldn't have problems treating the child of your stepmother that way, if you were worried about him or you thought he was embarrassing the family.
Nothing in that bible passage requires these to be the children of Mary. Only the children of Joesph or Jesus' kinsmen.
Sometimes, we have to just to agree to disagree on that point, then move on to something else that will help us both grow as Christians.
God bless.