Another explanation is that Cleophas was the brother of Joseph (Mary's husband) and Cleophas wife was also named Mary, meaning they were sisters-in-law, not biological sisters. Mary's biological sister was thought to be Salome, who was Zebedee's wife, the mother of James and John. This would make James and John the cousins of Jesus--and Peter, as a fisherman, knew and worked with James and John. Andrew, of course, was Peter's brother.
It appears that his mother, two aunts, and Mary Magdalene were the women at Jesus' crucifixion, and were also the ones preparing for his burial. (Mark mentions Salome by name at the crucifixion.)
When Jesus told John, "Behold your mother..." and Mary, "Behold your son..." the two had a blood relationship of aunt and nephew.