“Hermanos” (brothers) is still used in Spanish to indicate brothers and cousins in a family. The full title of a cousin is “primo hermano” (abbreviated to primo). It is not and was not unusual in many languages for the title “brother” to encompass all male relatives who were roughly in the same age cohort.
If you think the word "cousin" is confusing you should read Jeromes' 11 page explanation about the word "until" in Matthew 1;25.
Chinese has some similar customs.
But that’s not my point.
My point is to build such a critical dogma on such flimsy evidence is horrendously silly.
I don’t know why God didn’t emphasize the blood sibling relationship but He didn’t. NEITHER DID HE MAKE CLEAR IN SCRIPTURE the interpretation the Roman edifice layers on it.
Therefore, building a supposedly infallible dogma on such flimsy a foundation is . . . spiritual, religious malpractice of the worst kind, imho.