Well, scripture says, “Joseph...took his wife, but knew her not until she had given birth to a son.”
It says, “And she gave birth to her firstborn son and wrapped him in swaddling cloths...”
And it says, “And his mother and his brothers came, and standing outside they sent to him and called him. And a crowd was sitting around him, and they said to him, “Your mother and your brothers are outside, seeking you.” And he answered them, “Who are my mother and my brothers?” And looking about at those who sat around him, he said, “Here are my mother and my brothers! “Whoever does the will of God, he is my brother and sister and mother.”
and, ““Is not this the carpenter, the son of Mary and brother of James and Joses and Judas and Simon? And are not his sisters here with us?” And they took offense at him.”
If you want to claim Jesus was an only child, it seems the burden of proof is on you. And FWIW, both Greek and Aramaic had words for cousins or kinfolk.
The "here are my mother and brothers" is a figure of speech, and is no more to be taken literally than, as C.S. Lewis said, when St. Paul said we were to be as doves he wanted us to lay eggs.
And 'until' in the KJV does not necessarily imply that the negative eventually happened. When it says that "Samuel came no more to see Saul until the day of his death," it means that he never saw him again, not that he showed up the day he died.
References please.
Neither Hebrew nor Aramaic (the language spoken by Christ and his disciples) had a special word meaning "cousin," speakers of those languages could use either the word for "brother" or a circumlocution, such as "the son of my uncle." But circumlocutions are clumsy, so the Jews often used "brother."
The writers of the New Testament were brought up using the Aramaic equivalent of "brothers" to mean both cousins and sons of the same father—plus other relatives and even non-relatives. When they wrote in Greek, they did the same thing the translators of the Septuagint did. (The Septuagint was the Greek version of the Hebrew Bible; it was translated by Hellenistic Jews a century or two before Christ’s birth and was the version of the Bible from which most of the Old Testament quotations found in the New Testament are taken.) In the Septuagint the Hebrew word that includes both brothers and cousins was translated as adelphos, which in Greek usually has the narrow meaning that the English "brother" has. Unlike Hebrew or Aramaic, Greek has a separate word for cousin, anepsios, but the translators of the Septuagint used adelphos, even for true cousins.