The "Adelphos doesn't mean Jesus had blood siblings" argument falls apart with this verse.
Galatians 1:19 I saw none of the other apostles—only James, the Lord's brother.
If he was using the term as bretheren, as in for the apostles as has been forwarded before, there wouldn't be the distinct classifier of the Lord's brother. It would read I saw none of the other apostles, only James. (no need to add the honerific adelphos.)
Some will argue that it meant cousin. However, if you check out this verse:
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International Standard Version (©2008)
Aristarchus, my fellow prisoner, sends his greetings, as does Mark, the cousin of Barnabas. You have received instructions about him. If he comes to you, welcome him.
New American Standard Bible (©1995)
Aristarchus, my fellow prisoner, sends you his greetings; and also Barnabas's cousin Mark (about whom you received instructions; if he comes to you, welcome him);
GOD'S WORD® Translation (©1995)
Aristarchus, who is a prisoner like me, sends greetings. So does Mark, the cousin of Barnabas. You have received instructions about Mark. If he comes to you, welcome him.
King James Bible
Aristarchus my fellowprisoner saluteth you, and Marcus, sister's son to Barnabas, (touching whom ye received commandments: if he come unto you, receive him
American King James Version
Aristarchus my fellow prisoner salutes you, and Marcus, sister's son to Barnabas, (touching whom you received commandments: if he come to you, receive him
American Standard Version
Aristarchus my fellow-prisoner saluteth you, and Mark, the cousin of Barnabas (touching whom ye received commandments; if he come unto you, receive him),
Bible in Basic English
Aristarchus, my brother-prisoner, sends his love to you, and Mark, a relation of Barnabas (about whom you have been given orders: if he comes to you, be kind to him),
Douay-Rheims Bible
Aristarchus, my fellow prisoner, saluteth you, and Mark, the cousin german of Barnabus, touching whom you have received commandments; if he come unto you, receive him:
Darby Bible Translation
Aristarchus my fellow-captive salutes you, and Mark, Barnabas's cousin, concerning whom ye have received orders, (if he come to you, receive him,)
English Revised Version
Aristarchus my fellow-prisoner saluteth you, and Mark, the cousin of Barnabas (touching whom ye received commandments; if he come unto you, receive him),
Webster's Bible Translation
Aristarchus, my fellow-prisoner, saluteth you; and Marcus, sister's son to Barnabas, (concerning whom ye received commandments: if he should come to you, receive him
Weymouth New Testament
Aristarchus my fellow prisoner sends greeting to you, and so does Barnabas's cousin Mark. You have received instructions as to him; if he comes to you, give him a welcome.
World English Bible
Aristarchus, my fellow prisoner, greets you, and Mark, the cousin of Barnabas (concerning whom you received commandments, "if he comes to you, receive him"),
Young's Literal Translation
Salute you doth Aristarchus, my fellow-captive, and Marcus, the nephew of Barnabas, (concerning whom ye did receive commands -- if he may come unto you receive him,)
you can see that indeed the translation to Cousin is used, and understood. Not one translates to "Brother, despite the adamant assertion that they had no word for it. Context is important. I dare say that if they are able to identify the relationship as Cousin, they would do so. They did in this instance.
(Either that, or the author was slightly cracked, of course, but that's a third choice that I find highly unlikely.)
One would have to explain why it was clearly cousin in the above passage, but the others are left vague (if indeed the PV is accurate, and Jesus had no blood siblings.)