*]"But there was a Greek word for cousin, anepsios. If the brothers of the Lord were really his cousins, why wasn’t that word used?"
Here is a common misconception to be on the lookout for: “Catholics teach that the brothers were actually cousins.” That’s not the Catholic position. In fact, we can’t tell if any of the “brothers” were cousins. All the Church affirms is that they were not children of Mary. They could have been children of Joseph from a prior marriage. But the specific word for cousin (anepsios) probably would not have been used in Matthew 13:55 unless all the “brothers” were cousins. If even one of them was not a cousin, the more general term " adelphoi" covers the situation. Even if all of them were cousins, the term “brother” could still be used by Matthew to appropriately describe them.
These things were taken for granted by the early Christians, who were familiar with the biblical languages and who knew that Mary was a lifelong virgin. In A.D. 380, Helvidius proposed that Mary had other children because of the “brothers” in Matthew 13:55. He was rebutted by Jerome, who was arguably the greatest biblical scholar of the day. The Protestant reformer John Calvin seconded Jerome: “Helvidius has shown himself too ignorant, in saying that Mary had several sons, because mention is made in some passages to the brothers of Christ” [quoted by Bernard Leeming, *Protestants and Our Lady, 9]
. Martin Luther agreed with Calvin that Mary was always a virgin, as did Ulrich Zwingli: “I esteem immensely the Mother of God, the ever chaste, immaculate Virgin Mary” [E. Stakemeier, *De Mariologia et Oecumenismo