"Woman, behold your son. Son behold your mother."

Jesus, the only son of Mary, knows that when he dies, Mary, as a widow, will have no immediate family member to care and to provide for her in her material needs. The law required the firstborn son to take care of his parents, and Jesus was obeying the law of God up until the end. In his great love for his mother, Jesus would not commit his soul to his Father without seeing to the care of his mother. Thus, Jesus entrusts her to John and John to her. Jesus, in a sense, utters his last will and testament concerning the providential care of his mother.

The Church proclaims Mary, ever Virgin. She was a virgin prior to conception, during and after the birth of Jesus. Why would Jesus entrust her to someone not blood-related, unless she had no other living blood relations?  

Jesus was doing the Father's will, offering his life as a sacrifice for our sins. At the same time, he was doing the Father's will, providing for his mother.  We are called to do the Father's will in our relationships especially with those closest to us.