A Roman Catholic writes:
We are required to believe in Mary's perpetual virginity, meaning that she was always a virgin and had no other children, and that Jesus' birth was a miraculous one, not (far as I know without checking) in any particular hypothesis accounting for the exact nature of the relationship of these persons called Jesus' "brothers" in Scripture, according to standard Hebrew / Aramaic cultural custom.
The problem with the Aramaic/Hebrew/cousins argument is that the NT books were written in Greek; and there is a specific word for "cousin" in Greek, used in Colossians 4:10
Μᾶρκος ὁ ἀνεψιὸς Βαρναβᾶ
Mark the cousin of Barnabas
cousin = anepsios (English phonetics)
If they were cousins, the NT writers would have used the word for “cousin”.
Also, there are other words for close relative, cousin, as in Luke 1:36 "your relative Elizabeth"
Ἐλισάβετ ἡ συγγενίς
sungenis (English phonetics) The double gamma, "g" is pronouned "ng".
Also, in Matthew 12:49-50 - the point would be lost, if he meant "cousins", because Jesus makes the point that His true spiritual brothers are disciples, believers; and His blood (half) brothers were not believers at that time. ( John 7:5 "for even His brothers were not believing in Him.") This point, Tertullian argues in Against Marcion 4:19. He is more ancient than the later "ever-Virgin" texts of other early church fathers; so this position, that Mary and Joseph had a normal marriage after Jesus was born, is the one that is deeper in history.
In Hebrews 2:10-11, it says, ". . . for which reason His is not ashamed to call them brothers." (those who are sanctified, the believers, who trust Him (v. 13)
In Matthew 12:50 - He is making this point, the true brothers are those that believe and obey and do the will of God; not His physical brothers who don't believe.
Galatians and Corinthians were Greek/Gentile areas, they did not speak Aramaic, so even more reason for those books to use the word "cousin" (anepsios) if James was his cousin. But no, he calls him "the Lord's brother". Galatians 1:19
The whole RC argument for the Perpetual Virginity of Mary falls flat.
Also, Psalm 69:8-9 seems to point to His brothers being His mother's sons. Psalm 69 has lots of Messianic prophecy in it; though not all of it applies to Christ, obviously verse 5 does not apply to Jesus.
"I have become estranged from my brothers, and an alien to my mother's sons. For zeal for Thy House has consumed Me, and the reproaches of those who reproach Thee have fallen on Me." (see John 2:17; Romans 15:3; and also John 15:25)
Hebrew parallelism "brothers" = "mother's sons"
Obviously, Psalm 69:5 is NOT about Jesus, because He was sinless, but one cannot deny that there are many other verses in Psalm 69 that are Messianic and many quotes from this Psalm are about Jesus Himself. (see also Psalm 69:21 (in John 19:28-30)
Another Roman Catholic commenter wrote:
If Mary had male children other than Jesus, it would have been a HUGE insult to them for Jesus to entrust her care to the apostle John who was not a "blood" relation.
Jesus does not mind offending people. His brothers were already offended and insulted by Jesus in Mark 6:3. This is not a big deal for Jesus to offend His brothers, since they were unbelievers and, again, He is not ashamed to call believers, His brothers. Heb. 2:11
So, He is ashamed to call His real brothers, "brothers" (at the cross) and does not honor them with the care of Mary; but gives her to a faithful believer, John.
So, giving His mother to John, a believer, a true disciple, a true spiritual brother, was the right thing to do, considering his physical brothers were not believers at that point. They became believers at or after the resurrection. (I Cor. 15:7; Galatians 1:19)
So, Jesus is not ashamed to call John his brother from the cross over His physical half-brothers, shaming them, because of their unbelief. (see also Matthew 10:32-40)
Eric Svendsen's book, Who is My Mother? Soundly and thoroughly refutes the RC apologetic for the Perpetual Virginity of Mary.
Rhology had a post on the PVM also some time ago. He provides some good links to further refutations of the PVM.