In other words, the box inscription counts against both the Roman Catholic cousin theory and the Orthodox theory that they are children of Joseph by prior marriage.
Assuming the box isn't a forgery:
First, you should note that the inscription says son of Joseph, not son of Mary.
Second, the names James, Joseph, and Jesus were very common during this period and you would have to prove the names on the box refer to the people of interest. When this box came out, there was an analysis done based on the population of Jerusalem at the time of about 80,000 and the commonality of these names and it was estimated about 20 people could be called "James son of Joseph brother of Jesus."
Third, you'd have to prove the word brother, as used here, meant from the same womb.
But alas it is not proven to be a forgery. By your logic then the Shroud of Turin is a forgery - scientific dating has shown it to be of Renaissance dating. Should the Catholic Church be arrested for perpetrating this fraud on the world? The ossuary on the other hand was tested and the patina was shown to has been of first century origin.