I know you've been waiting to try this out on some unsuspecting Sola Scripturist, but we've been down this road many times before.
Let me cut through the chase and get to the meat of the question. Did the word "brethren " in Matthew 13:55, mean they all had the same mother, or could it have meant cousin, or a half brother, or a spiritual brother?
First, look at Matthew 13:55-56, and I believe you'll have to agree that this is Jesus family they're referring to.
Is not this the carpenter's son? is not his mother called Mary? and his brethren, James, and Joses, and Simon, and Judas?
V- 5. And his sisters, are they not all with us? Whence then hath this man all these things?
From this we know that Jesus father was a carpenter, that His mothers name was Mary. We also know that Jesus had brothers ranging from the oldest, James then Joses, Simon and Judas, plus a couple of sisters.
If you'll look at Matthew 27:56 at the scene of the crucifixion, you'll find this same Mary at the scene of the crucifixion, and the only Mary besides Mary Magdalene, whos at every accounting at the cross.
Mt 13:55, Mk 15:40, Jn 19:25, these two Marys are inseparable. They are always together, so they must be followers of Jesus, and more then passing friends.
Now notice that Mary has sons, shes their mother.
Unlike some who say that Jesus had brothers but it never says that Mary was their mother, well thats not true, because here at the crucifixion, all the writers except John, tell us Mary was the mother of these sons and brothers of Jesus, James and Joses, the same James and Joses called Jesus brethren in Matthew 13:55.
John doesnt use the two sons James and Joses to clarify who Mary was, because he made it clear she was Mary, the mother of Jesus.
Its clear that Mary had sons named James and Joses, and she was the only Mary in the New Testament who had sons with either of these names.
This was not a large group of followers, who would have been at the cross with Jesus, so its absurd to think one other Mary out of this small company of people, could also have had two sons named James and Joses, and a carpenter father.
Had there been, the writer (and the Holy Spirit) would have clarified which was which.
Just as I challenged Soothing Dave to find me another woman named Mary who had sons named James and Joses, I also challenge you, and please dont bother me again trying to say Mary remained ever virgin unless you can.
JH