How do we know that Mary was the aunt of John whose mother was Salome?
It’s plausible, based on parallel accounts of Christ’s crucifixion. But we’re not certain.
In Matthew 27:56, there is a woman described as the “mother of the sons of Zebedee,” who are elsewhere identified as James and John (Matt. 10:2; 20:20ff.). In the parallel synoptic Gospel account of Mark, this person is identified as Salome, and thus Salome is understood to be the mother of the sons of Zebedee.
In John’s account of the crucifixion, neither the mother of the sons of Zebedee nor Salome is mentioned. However, Jesus’ mother and “his mother’s sister” are (John 19:25). Was this another person or Salome, the mother of the sons of Zebedee? We’re not certain. If the latter, this would show that Salome is Mary’s sister, whether directly or as a cousin/relative. And thus Mary would be the aunt of James and John, the sons of Salome, her sister.
For more information, see this article on Salome and also our tract “The Brethren of the Lord.”