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

Ἐλισάβετ ἡ συγγενίς
sungenis (English phonetics) The double gamma, "g" is pronouned "ng".
Also, in Matthew 12:49-50


In Hebrews 2:10-11

In Matthew 12:50

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

"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



Hebrew parallelism "brothers" = "mother's sons"
Obviously, Psalm 69:5



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


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


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.
http://beggarsallreformation.blogspot.com/2009/02/marriage-bed-is-defiled.html