I don't even mean to go so far. Paul can say "anepsios," because he is writing his own words to a Greek audience. He specifically means a cousin, and so he chooses a word specifically meaning cousin.
Matthew is reporting what was spoken by Jews. Since the Jews don't have a word for cousin, Matthew isn't going to report that the Jews called them cousins. The Jews said the Jewish word for brothers, so when Matthew writes in Greek, he uses the Greek word for brothers.
If we look at JUST that passage, we still don't know whether Jesus had cousins or brothers. Certainly "adelphos" does refer to actual brothers as well. I just set about to prove that people named James, Judas and Joses were in fact Jesus' cousins.
I have heard the argument over the word "until." The classical Greek word has been used to mean actions which did not necessarily stop after the condition was met, both before and after the writings of the gospel. I read one apologist, posted on this site, who made the strange claim that the gospel was written during a period where the word "until" was not so used; Catholics pointed out other uses of the word in the New Testament; the person who posted the original article claimed that conditions hadn't necessarily been meant afterward, and the whole thing became quite absurd.
“Behold My mother and My brothers! For whoever does the will of God, he is My brother and sister and mother.”
If the word “brother” meant “cousin” what does “sister” mean within this context? I believe He is referring to His immediate family (notice there is no mention of “father”). It doesn’t make sense and its inconsistent that our Lord Jesus would mean cousin in one case and brother and sister in another.
Tradition going back to Papias, cited by Eusebius, indicates that Matthew was originally composed in "Hebrew" (which may mean Aramaic), not in Greek.
There are oddities in parts of Luke, too -- word plays and anagrams which only appear when the Greek is translated into Hebrew. There's also a Hebraicism used several times in Luke called the "apodotic vav," which isn't typical of Greek, but might well appear in a text very carefully translated from idiomatic Hebrew into Greek.