His Mother

When Jesus told His mother, “behold your son” (John 19:26), to whom was He referring?
"The disciple whom Jesus loved” (John 19:26), which is John, the gospel writer, who was standing by her.

What was the relationship between His mother and John?
His mother was John's aunt, and John was His mother's nephew. That means John and his brother James were not only Jesus' disciples but also Jesus' half-cousins.

How do we know this?
John and James' father was Zebedee: "Now the names of the twelve apostles are these: first, Simon, who is called Peter, and Andrew his brother; James the son of Zebedee, and John his brother..." (Matthew 10:2). Later in the Gospel of Matthew, the women near the cross are identified in the same context as in John 19 above, but with slightly different information: "among whom were Mary Magdalene, Mary the mother of James and Joses, and the mother of Zebedee’s sons" (Matthew 27:56). Mary Magdalene is mentioned in both accounts. The two other women are described as (A) "Mary the mother of James and Joses" and (B) "mother of Zebedee’s sons" in Matthew 27:56, but (1) "His mother's sister" and (2) "Mary the wife of Clopas" in John 19:25. "Mother of Zebedee's sons" means the wife of Zebedee, which means she can't be "the wife of Clopas." Therefore, (B) wasn't (2), which means that (B) "the mother of Zebedee's sons" was (1) "His mother's sister." This may be why she had felt at liberty to ask for special favors from Jesus: "Then the mother of Zebedee’s sons came to Him with her sons, kneeling down and asking something from Him. And He said to her, “What do you wish?” She said to Him, “Grant that these two sons of mine may sit, one on Your right hand and the other on the left, in Your kingdom” (Matthew 20:20-21).

So when Jesus told John to "behold your mother" (John 19:27), what was He saying?
He was telling John to take care of his aunt like his own mother.

What kind of shape must Mary have been in at this point?
Devastated, as any mother would be at seeing her son reduced to a bloody pulp and nailed to a cross, just as she had been told (see Simeon).

What did John do thereafter?
"And from that hour that disciple took her to his own home" (John 19:27), so Jesus may have motioned to John with His eyes and/or head to take her away to save her from the pain of watching the rest of His suffering and death.

Why would Jesus send her to John's place when she had other sons?
After His resurrection, Jesus will show Himself first to His disciples, including John, so Mary's sorrow will turn into joy earlier by staying with John.