People always debate about whether Jesus had any brothers and sisters. The film Dogma picks up on this and explains how Bethany is the great-great-grandniece of Jesus.
The Gospels themselves make reference to Jesus' brothers. Which makes you wonder why people who start this debate don't bother reading the Book first.
Luke:
Jesus' Mother and Brothers
19Now Jesus' mother and brothers came to see him, but they were not able to get near him because of the crowd. 20Someone told him, "Your mother and brothers are standing outside, wanting to see you."
21He replied, "My mother and brothers are those who hear God's word and put it into practice."
Mark:
Jesus' Mother and Brothers
31Then Jesus' mother and brothers arrived. Standing outside, they sent someone in to call him. 32A crowd was sitting around him, and they told him, "Your mother and brothers are outside looking for you."
33"Who are my mother and my brothers?" he asked.
34Then he looked at those seated in a circle around him and said, "Here are my mother and my brothers! 35Whoever does God's will is my brother and sister and mother."
Matthew:
Jesus' Mother and Brothers
46While Jesus was still talking to the crowd, his mother and brothers stood outside, wanting to speak to him. 47Someone told him, "Your mother and brothers are standing outside, wanting to speak to you."
48He replied to him, "Who is my mother, and who are my brothers?" 49Pointing to his disciples, he said, "Here are my mother and my brothers. 50For whoever does the will of my Father in heaven is my brother and sister and mother."
Okay, so Matt pretty much says what Mark said. We knew he would.
Jesus is oddly dismissive of his family. His brother's may be Mary's sons' but they are also Joseph's son's not the sons of IHVH.
John strangely omits this story altogether. He probably had his reasons.
But, anyway, if someone brings up the issue of Jesus having brothers and sisters - tell them to read the Gospel.