Wonderful question came out of our Bible study tonight concerning the Apostle Thomas.
Since both Thomas (Greek) and Didymus (Aramaic) mean twin, and the Gospel of John refers to Thomas as "Thomas called Didymus", is this person the half-brother of Jesus that some historians claim?
If so, could it have been Thomas that Jesus declared "Woman, behold thy son! " and "Behold thy mother!" at the foot of the cross?
What say you?

Well done. Great argument and apologetic.
I don’t think that Thomas was a half brother of Jesus. We do not see any account of his brothers or any other family believing Jesus was the Messiah until Acts. Also, later in Paul’s letter to the Galatians(1:19) Paul lists James as the Lord’s brother. In the creed in 1 Corinthians 15, James is listed as one of the eyewitnesses to which Jesus appeared. In Mark, Thomas was called by Jesus to be an apostle. Mark is actually Peter’s account so this is coming from a very reputable source. Also, the early 1st Century Jewish historians, Josephus in particular always identified James as “the brother of Jesus” and did not with Thomas.
If Thomas was the half brother of Jesus, especially as he was one of the twelve apostles, it would have been made clear and scripture does not make that clear to me.
Mark 6:3 Jesus was teaching in the synagogue in Nazareth, his home town. The men in attendance asked regarding Jesus, "is not this the brother of James, Joses, Simon and Juda?"
James being listed first is most likely the eldest after Jesus and out of the four should have taken the responsibility of caring for his mother but Jesus asked his beloved disciple to take care of her.
Thomas was called by Jesus to be one of the twelve apostles recorded in Mk. 3:13-18; Matt 10:1-3; Lk. 6:12-16. In Mark 4:10 there is also a reference made of the disciples with the twelve, putting a distinction between the two groups.
In Matt.12:46-47; Lk. 8:19-20, and Mk 3:31-32 it is stated that "thy mother and brethren stand without desiring to speak to thee". The twelve were already chosen and most likely with Jesus inside.
In John 7:3,5 It states, "For neither did his brethren believe in him". We do not have record of Jesus' half brothers believing in Him as the Messiah until the book of Acts.