Matthew 1:25 implies that Joseph and Mary had normal sexual relations after the birth of Jesus. Those sexual relations likely resulted in children. The later references in Scripture logically refer to the brothers and sisters of Jesus are most likely the products of those sexual relations. Bearing other children in natural, lawful sexual relations with her husband, doesn't diminish the holiness of Mary. IMO the doctrine of the Perpetual Virginity of Mary is based on poor hermeneutics.
In a Semitic society then as now, the care of the mother is with her sons. One does not entrust one's mother to the care of someone who is not her son or daughter. It is not done.
Since when is the Lord of All restricted by man-made cultural restraints? I don't claim to know Jesus' motivation for elevating John to the position of son above his other half-brothers who were most likely the sons of Mary but He did.
In the same sense Mary and Joseph who were already 'espoused' according to luke 1:27 which is why he wanted to "divorce her quietly" -- there would be no need to do that if he was not already married to her.
as scripture says, Jesus followed the Law and customs. He would not hand over custody of His mother to a non-son of Mary's