It is interesting that the Coptic Synaxarium also points to Joseph being a widower, and therefore follows implicitly the tradition that the brethren are sons by this first marriage. It says..
On this day, the righteous man St. Joseph, the carpenter, who was worthy to be called the father of Christ in the flesh, departed at a good old age. The Holy Gospel bore witness that he was a righteous man, and God chose him to be betrothed to the all-pure, our lady, the Virgin St. Mary. When he finished his course, his strife, his toil in the journey together with the Lord and the Virgin Lady from Bethlehem to the land of Egypt, and the tribulations that befell him from the Jews, he departed in peace. When the time came for him to depart from this world, to the world of the living, the Lord Christ was present at his departure, and laid His hand upon his eyes. He extended his arms, and delivered up his soul, and was buried in the tomb of his father Jacob. All the days of his life were one hundred and eleven years; forty years before his marriage, fifty-two years married, and nineteen years a widow. His departure was in the sixteenth year of the advent of the Lord Christ.
Certainly St Joseph predeceased the Virgin Mary, and therefore if he was 19 years a widow according to the Synaxarium then he must have been previously married.
What is interesting is that an older Arabic version of the Synaxarium in the Patrologia Orientalis series adds additional material that is not present in the modern English version. It is added..
And when the time came for him to pass from this world to the world of life, he called his four sons, Youstos, Yahouda, Yousab and Yacoub, and his three daughters. He made his recommendations and farewells to them, stretching out his hand and giving up his soul. The total length of his life were one hundred and eleven; until his marriage there were forty years, he was married for fifty-two years, and widowed nineteen years, three years before the incarnation of the Messiah.
This passage has been removed from the Arabic translation into English. It seems very clearly to describe the children of Joseph being gathered around him when he reposed. There is no reason for his nephews and nieces to do so.
It would seem to me that this material from the Arabic texts used in the early years of the 20th century suggests that any change to the tradition of the Church concerning these brethren has taken place after that date and within the 20th century.