The first readers of the Gospels would have known immediately who this third Mary was and why John identified her with her husband. Since there were three Marys at the cross, John needed to name the third Mary by her husband, Cleopas.
However, there may have been another reason for John to refer to her as the wife of Cleopas. According to tradition, Cleopas was Joseph’s younger brother, (Mary’s husband Joseph). What some also believed is that Mary, Cleopas’ wife, was Joseph’s daughter by his first wife. Hence, the two Marys were sisters-in-law, or as the Gospels call them, “sisters.”
While there is some dispute among scholars as to the exact relationship between the Marys, there is little question that they were related in some way. If we go with the notion that the Marys were sisters-in-law, that would have made Mary, wife of Cleopas, the aunt of Jesus.
Now clearly, these are only theories, and cannot be proven from scriptural references. However, comparing scriptural references and combining them with the writings of early church fathers allows us to construct a possible arrangement of relationships.
Mary Cleopas was described as the mother of James and Joseph (Matthew 27:56) in one account, and mother of James the Less and Joses in another (Mark 15:40). On the other hand, James is described as the son of Alphaeus in the synoptic Gospels' listing of the Apostles (Matthew 10:3, Mark 3:18, Luke 6:15). We can infer that Mary wife of Cleophas is unlikely to be a true sister of the Mary, the mother of Jesus, since they bear the same name.
The ancient historian, Hegesippus finished his Memoirs in the reign of Pope Eleutherius (AD 175-189) when he was an old man. He drew on information from personal sources, and was able to question some surviving members of Jesus' family. He related that:
"After the martyrdom of James, it was unanimously decided that Simeon, son of Clopas, was worthy to occupy the see of Jerusalem. He was, it is said, a cousin of the Saviour;" Hegesippus then recounts that Clopas was a brother of Joseph (Eusebius, Hist. eccl., III, 11).
St. Epiphanius reports the same thing and adds "that this Simeon, the son of Clopas, was a cousin of James the Just," as Hegesippus says in another passage.
Since Cleophas may have been the brother of Joseph (Jesus' adopted father), it follows that Cleophas' wife Mary would be the sister in law of Jesus' mother Mary, explaining why they can have the same name and are called sisters.
This is all an interesting historical mystery, and can be entertaining to pursue. However, it is, quite happily, not an essential issue, which is to say that the true facts about Mary Clopas and her relationships are irrelevant to our salvation.
