Catholics insist on a lot of things that are not true. The Greek word for cousin is ἀνεψιὸς (anepsios) as used in Colossians 4.
Colossians 4:10 My fellow prisoner Aristarchus sends you his greetings, as does Mark, the cousin of Barnabas.
The Greek word for brother used in Matthew 12:46 and other places referring to Jesus siblings is ἀδελφοὶ (adelphoi). It's also used here.
Mark 12:20 Now there were seven brothers. The first one married and died without leaving any children.
Do you seriously think that when the Holy Spirit inspired the writing of scripture that He was unaware of the difference in the two words? The Holy Spirit chose the Greek for a purpose. Simply repeating the Catholic Church attempts to justify the perpetual virginity of Mary only make people to appear cultish.
The Catholic church is repeating the testimony of the Church fathers that the Blessed Virgin remained ever-virgin. THAT’S the Catholic doctrine.
Anepsios is a Greek word with no Aramaic equivalent. Colossians was written in Greek, by someone who spoke Greek, writing to Greeks. Mark is quoting Jesus, who spoke Aramaic.
Greek Orthodox doesn’t like the notion that there could be any distinction between the gospels’ translations into Greek and native Greek. Therefore, they resolve the notion that Mary was ever-Virgin and Jesus had “adelphoi” by insisting that Joseph was a remarried widower; therefore Jesus would have older half-brothers. Catholics find no historical basis for this, so it’s not their preferred way of resolving this, but they do not regard the Greek notion as heretical. They do regard the notion that Jesus had younger brothers as contrary to the doctrine of the perpetual virginity of Mary.
You’ll notice, in fact, that many of these Catholic painters retain the notion that Joseph was elderly, in deference to the Greek notion that he was a widower.