Concise Bible Dictionary:
This name occurs but once, John 19:25; it is really “Clopas”; the word “wife” which is added in the AV is probably correct. By comparing together the following passages (Matt. 27:56,61; Matt. 28:1; Mark 15:40,47; Mark 16:1); and Luke 24:10, it appears probable that “the other Mary,” and “Mary the mother of James (or James the less) and Joses” allude to the same person, and she may have been the wife of Clopas. In John 19:25 this Mary is said to be the sister of the Lord’s mother, if we allow the word “and” to separate the persons, which is the most obvious sense; and it is natural that as this is the only place where we read of the Lord’s mother having a sister, it should say who she was. Thus there would be three Marys mentioned in the verse, and not four. That the Lord’s mother should have a sister also called Mary may appear, improbable, but the MSS vary, and there may have been a slight difference, as in the two modern names of Mary and Maria.