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Nor, far more important to the conjecturist

You wisely use the word conjecturist, because that is the most anyone can be regarding the brother and sister passages. There just is not sufficient information from Scripture to strongly support whether Mary did have other children or whether Mary did not have other children.

In contrast to your argument, here is an argument from Ellicott. Neither argument proves the point.

It is a slight argument in their favour, (1) that it would have been natural had there been other children borne by the mother of the Lord, that the fact should have been recorded by the Evangelists, as in the family narratives of the Old Testament (e.g., Genesis 5, 11; 1 Chronicles 1, 2), and that there is no record of any such birth in either of the two Gospels that give “the book of the generations” of Jesus; (2) that the tone of the brethren, their unbelief, their attempts to restrain Him, suggest the thought of their being elder brothers in some sense, rather than such as had been trained in reverential love for the first-born of the house; (3) that it is scarcely probable that our Lord should have committed His mother to the care of the disciple whom He loved (John 19:26) had she had children of her own, whose duty it was to protect and cherish her; (4) the absence of any later mention of the sisters at or after the time of the Crucifixion suggests the same conclusion, as falling in with the idea of the sisters and brethren being in some sense a distinct family, with divided interests; (5) lastly, though we enter here on the uncertain region of feeling, if we accept the narratives of the birth and infancy given by St. Matthew and St. Luke, it is at least conceivable that the mysterious awfulness of the work so committed to him may have led Joseph to rest in the task of loving guardianship which thus became at once the duty and the blessedness of the remainder of his life. On the whole, then, I incline to rest in the belief that the so-called “brethren” were cousins who, through some unrecorded circumstances, had been so far adopted into the household at Nazareth as to be known by the term of nearer relationship.

https://www.studylight.org/commentary/matthew/28-20.html