First, it was not important to write Mary's biography
If it wasn't important at that time, then why later did it become so important as to merit "Holy Tradition"?
Everything else is only sketched as necessary or not mentioned at all.
Apparently it was germane to Matthew and so was mentioned. What is disturbing here is that what is contained in the Sacred Scriptures are described by the RCC as inaccurate "sketches". Since Matthew 1:24-25 said that Mary got laid by her husband and had other sons, and "Holy Tradition" contradicts that statement, then this portion of divinely inspired writing gets tossed into the waste bin of erroneous "sketches" and is replaced by the "unimportant" doctrine of Mary's sin against God in refusing the role of wife. (1 Corinthians 7:2-5; Matthew 19:5) What is clear here is the belief that "Holy Tradition", of suspicious origin and definition, has greater authority than the Divinely inspired Word of God. In other words, the utterances of men chosen by cast lots thousands of years beyond the events have greater authority than the words of eye-witnesses of and those who were direct understudies of Jesus Christ Himself.
There was a reason why Rome burned down so many libraries and killed so-many Church leaders that differed from her extra-biblical proclamations. We see that reason here, as Matthew's Gospel is held in contempt.
Lastly, to say "brothers" is good writing in this context, even though it lends itself poorly to English translations in the society where there is one home per family and the family has 1.9 children.
I didn't know that in 1611 (the KJV's publication) that the average English speaking family had 1.9 children, I foolishly thought it was considerably greater. The problem I see with your theory, other than the fact cultures have indeed changed over time, is that there are good words to use for "cousin". A relevant example is here the word 'άνεψιός' which, means "cousin", or if you are a 1611 KJV translator, means "nephew". (compare Col 4:10 between versions).
Your "brothers" means "cousins" statement is hogwash. There is a perfectly good word to use in the context that you are constructing, and that word is 'συγγενίς', and that it is a malleable in meaning as needed, yet is commonly understood to be "cousin" or "kinsmen". I submit that the Gospel authors meant "brothers" and thus used 'άδελϕός' when they meant "brothers" rather than "cousins". Furthermore, and quite important, is the word used in Matthew 13:56 and that is for "sister" or 'άδελϕή', and it never means "kinsmen" or "cousins" but is either referring to natural sisters or ecclesiastical sisters.
My question for you is, if the Gospel writers wanted to convey the idea of actual brothers and sisters, what would they have actually said differently than what is already there?
I know the temptation to rewrite dictionaries is there because Rome has no problems rewriting history and the Word of God.
It was Holy Tradition all along. Facts of apostolic origin known to the Church are either the Holy Tradition or Holy Scripture. Tradition became scripture at some point, but nothing can become Tradition by definition of the word. One can, of course, develop everyday traditions, like when to fast etc. Further, since life goes on, lives of saints get incorporated in the traditions of the local Church. But since Mary belongs to the age of the Apostles, the facts of her life are part of the same deposit of faith given the Apostles as the entire Holy Tradition "once delivered to the saints" (Jude 1:3).
Apparently it was germane to Matthew and so was mentioned [in Mt 1:24-25]
It is not in Matthew 24-25. In Matthew 1:24-25 we are told that prior to Christ's birth Mary had no physical relations with Joseph and that Jesus was the first male child that she had, the firstborn. It did not say that Mary subsequently had any physical relations with Joseph, or had other children, or retired in a condo in Florida, or anything else. You are adding your fantasies to the Holy Scripture.
Mary's sin against God in refusing the role of wife. (1 Corinthians 7:2-5; Matthew 19:5)
Neither is the idea that Joseph asked for something she did not give him is in the scripture. Again, like with the entire Protestant baloney on sticks, the scripture is not read for what it says, but rather isolated verses are twisted till they fit in the preconceived notion of what you think the scripture should be saying. The comical part is that this process of distortion is called "Scripture Alone".
that there are good words to use for "cousin". A relevant example is here the word 'άνεψιός' which, means "cousin", or if you are a 1611 KJV translator, means "nephew". (compare Col 4:10 between versions).
I don't think I said, or St. Matthew meant to say "cousins". Brothers is a correct term in the Greek of the period, for a collection of relatives who are close in age, whose precise relation is either not known or not important, or differs inside the group. To say, for example, "cousins" excludes step brothers, milk brothers adopted brothers and playmates.
wanted to convey the idea of actual brothers and sisters, what would they have actually said differently than what is already there?
If it were important, the would indicate who the parents were, like Mary Cleophas is indicated as the mother of some of these putative "brothers" (John 19:25, Mark 15:40).
Rome has no problems rewriting history and the Word of God.
What did Rome rewrite? I can tell you what Luther rewrote (Romans 3:28).