Catholics -> adelphos could mean ”brothers” but doesn’t in this case. It means “cousins”.
Non-Catholics -> adelphos could mean “cousins” but doesn’t in this case. It means “brothers”.
The push by Catholics to understand the text to mean “cousins” happens because of their dogma concerning Mary, not from the text. Absent the dogma the natural meaning of “brothers” is “brothers”. With the dogma there isn’t even the remotest possibility that “brothers” could actually mean “brothers”. The dogma interprets and controls the text.
So where does the dogma come from? It comes from the teaching of the Roman Catholic Church.
The ambiguity of the word adelphos is resolved, for a Catholic, by the tradition and authority of the Roman Catholic Church.
The issue then becomes that of either accepting or rejecting the validity of the tradition and teaching of the Roman Catholic Church. This is the overriding issue.
Non-Catholics have to understand that the fundamental issue here isn’t the range of meaning of adelphos (that’s easily agreed upon) but rather the authority of the Roman Catholic Church.
In order for a Catholic to agree with a non-Catholic on the question asked in this thread, a Catholic must reject the teaching authority of the Roman Catholic Church. In short, the Catholic must cease being Catholic.
The Catholic is essentially being “invited“ to leave, to reject, to renounce, the Catholic Church. That’s unthinkable to a devout Catholic. In the words one of my wife’s relatives, “the devil is seducing us, tempting us, seeking to destroy the Church [with the argument that Mary gave birth to other children]”.
That, in my personal experience, is what people on both sides of the question are really up against.