That Mary had no other children. Not once in the Bible do any of the pretend brethren or sisters refer to Mary as their mother, nor does anyone else in the entire Bible say that Mary is the mother of one of them.
Some see Psalm 69 as stating that Mary is the mother of one of them:
Psalms 69:7 Because for thy sake I have borne reproach;
shame hath covered my face.
8 I am become a stranger unto my brethren,
and an alien unto my mother's children.
9 For the zeal of thine house hath eaten me up;
and the reproaches of them that reproached thee are fallen upon me. (KJV)
Verse 7 and 9 are connected by the reproaches falling upon him, and the in the midst of that is verse 8 referring to the bretheren, mother's children.
Now it is true that there are verses in this Psalm that do not refer to Jesus. But verse 9 is applied in the NT to Jesus, and is connected to the theme of verse 7. And Jesus' was alienated from His brothers for a time, unjustly.
John 2:17 Then His disciples remembered that it was written, “Zeal for Your house has eaten Me up.” (NKJV)
Romans 15:3 For even Christ did not please Himself; but as it is written, “The reproaches of those who reproached You fell on Me.” (NKJV)
Augustine, while admitting that verse 8 refers to Jesus, when discussing Psalm 69, applies it differently:
ccel.org
12. “An alien I have become to My brethren, and a stranger to the sons of My mother” (ver. 8). To the sons of the Synagogue He became a stranger…Why so? Why did they not acknowledge? Why did they call Him an alien? Why did they dare to say, we know not whence He is? “Because the zeal of Thine House hath eaten Me up:” that is, because I have persecuted in them their own iniquities, because I have not patiently borne those whom I have rebuked, because I have sought Thy glory in Thy House, because I have scourged them that in the Temple dealt unseemly: John ii. 15. in which place also there is quoted, “the zeal of Thine House hath eaten Me up.” Hence an alien, hence a Stranger; hence, we know not whence He is. They would have acknowledged whence I am, if they had acknowledged that which Thou hast commanded. For if I had found them keeping Thy commandments, the zeal of Thine House would not have eaten Me up. “And the reproaches of men reproaching Thee have fallen upon Me.” Of this testimony Paul the Apostle hath also made use (there hath been read but now the very lesson), and saith, “Whatsoever things aforetime have been written, have been written that we might be instructed.” Rom. xv. 4. …Why “Thee”? Is the Father reproached, and not Christ Himself? Why have “the reproaches of men reproaching Thee fallen upon Me”? Because, “he that hath known Me, hath known the Father also:” John xiv. 9. because no one hath reviled Christ without reviling God: because no one honoureth the Father, except he that honoureth the Son also. John v. 23.