Considering the culture of the Jewish family at the time of Christ (and indeed today), why did Jesus give his mother into the care of a disciple (John) and not to one of His brothers and sisters? This act would have been the greatest insult to His family (brothers and sisters) had they existed. One other point: If Christ did have brothers and sisters, then there would be descendents of these alive today, brothers and sisters of the living God with sinful fallen natures.
There are separate words for “brother” and for “cousin” in the Greek. Jesus’ brothers are never called cousins in the New Testament. At the time of Jesus handing his mother over to the Beloved Disciple, his brothers did not believe in him (John 7:5). In any case, Mark 3:31-35 makes clear that Jesus saw the family of faith–not the physical family–as the primary family anyway.

The early church was quite clear about Jesus’ brothers being real brothers. This is one reason why James became the head of the Jerusalem church, and why later Jude was honored by having his brief letter included in the canon. We have no evidence that any of these children of Mary had offspring who had offspring in perpetuity (that is, up until the present). Finally, the text of Matthew 1:25 is quite clear–Joseph had no marital relationships with Mary until after Jesus was born. Thereafter, they did have such relationships, which produced children.
—Ben Witherington III