"Behold they son!... Behold thy mother!"

John 19:25-27. Now there stood by the cross of Jesus his mother, and his mother's sister, Mary the wife of Cleophas, and Mary Magdalene. When Jesus therefore saw his mother, and the disciple standing by, whom he loved, he saith unto his mother, Woman, behold thy son! Then saith he to the disciple, Behold thy mother! And from that hour that disciple took her unto his own home.

For a background summary and supporting argument, see Lazarus Writes: 

This is described by the eye-witness author of the Fourth Gospel - who was the resurrected Lazarus. Present with Lazarus at the foot of the cross stood Jesus's mother and aunt; and Mary Magdalene who was Jesus's wife; and Lazarus himself, brother to Mary M, and who was therefore Jesus's brother-in-law. 

This web-of-relationships explains why Jesus specifically asked his brother-in-law, the author of the Fourth Gospel, to "adopt" Jesus's mother after Jesus's death - Jesus's mother already being mother-in-law to Lazarus's sister. 

Jesus's wife's brother becomes the "son" to Jesus's mother; reciprocally, Jesus's mother became his brother-in-law's "mother".