The reason Jesus commissioned his beloved disciple John to take care of his mother, when normally this duty would pass to the next oldest son, I think is this: Jesus knew that each of his two brothers would soon be martyred themselves, and that John would live to the ripe old age of 90.
This is a possibilty but James the Just (Jesus’ bro) who was the first Bishop of Jerusalem lived another 28 years until he was murdered in 62AD. Maybe Jesus wanted to free him up from that responsibility to build the church.