There is an Aramaic word for cousin. I don’t remember it, but I found it once in an Aramaic dictionary.
And the NT was written in GREEK for a reason - that was a part of God’s plan. And Greek certainly had words for cousins, and it is used in the NT - but NEVER to describe one of the brothers of Jesus.
At a minimum, God gave imperfect guidance to the writers of scripture, if He allowed them to write what must otherwise obscure the truth. The plain language of scripture doesn’t lead one to believe Mary remained a virgin. So either God made it to confuse, or Mary didn’t remain a virgin.
Aramaic is a language going back four thousand years. It is very diverse since it served as the lingua franca of the Middle East for centuries, developing numerous dialects. Hypothetically, some dialect could have acquired a word for "cousin." But the fact that one dialect would have such a word--and that it might conceivably be found in some Aramaic dictionary would not prove that it could have been used in a scriptural context.