I don't see how this helps your case, but ok.
I find this a very weak argument simply because there is no reason for Matthew to have included that clause.
Duly noted. And if Matthew were a 20th Century American, I might agree with you. But he wasn't. He was a first century Jew and this kind of phrase was quite normal back then. There are other places in Scripture where this word is used in the same way and St. John gives one such example. But don't let the facts get in the way of a good argument. Perhaps we could return to this the next time the subject of using Scripture to interpret Scripture comes around.
Instead of:
"but kept her a virgin until she gave birth to a Son; and he called His name Jesus." Matt 1:25
it should have read:
"but kept her a virgin and he called His name Jesus."
Again, if he were a 20th century American, maybe so.
This coupled with the fact that
1) our Lord Jesus' "brothers" are defined in scriptures (Matt 12:46, 13:55, Mark 3, Luk 8, John 7, etc),
...discussed and refuted ad nauseum...
2) Paul refers to our Lord Jesus' "bothers" (1 Cor 9:5),
... same as above...
3) our Lord Jesus minimizes Mary's role to the role of all believers (Matt 12), and
...right or wrong, it's irrelevant...
4) the term cousin (often used as an argue as to what is meant here) is used in Col 4:10.
Translators can be funny that way.
I think the scriptural evidence is a bit overwheming.
If by "overwhelming" you mean "frustratingly elusive", I would have to agree.
Hmmm...let's summarize