As you said, it would be very odd for Christ to give Mary to John to take care of had He had other brothers. He would have been breaking Jewish law. In that day, “brothers” was used for cousins too. The term “firstborn” was a title of honor given to your first child, whether or not you had more. In Mt 13:55-56 four men are names as brothers of the Lord. However, at least two of them, James and Joseph, were the sons of Mary the wife of Cleophas. Also, when the family journeyed to the Temple and lost Jesus, no mention was made of other children or “brothers” of any kind.
There is more evidence that Christ was an only child than there is that He wasn’t .
“As you said, it would be very odd for Christ to give Mary to John to take care of had He had other brothers. He would have been breaking Jewish law.”
It is also very odd if Mary had step-sons who could take care of her. If a woman had no sons, her step sons should take care of her.
“In that day, “brothers” was used for cousins too.”
Yes, and even today we can use ‘brothers’ to speak of associates with no family relations. However, there WERE words for cousin, and they were commonly used. And nowhere in Scripture are they referred to as cousins. They were also always, while she lived, associated with Mary.
“The term “firstborn” was a title of honor given to your first child, whether or not you had more.”
Not exactly. When Luke wrote of John the Baptist, he didn’t call him ‘firstborn’. I’m not saying it is wrong, just that it wasn’t used by Luke when describing John.
“In Mt 13:55-56 four men are names as brothers of the Lord. However, at least two of them, James and Joseph, were the sons of Mary the wife of Cleophas.”
What it says is, “55 Is not this the carpenter’s son? Is not his mother called Mary? And are not his brothers James and Joseph and Simon and Judas? 56And are not all his sisters with us?”
The idea that James and Joseph were the sons of “Mary the wife of Cleophas” was first proposed by Jerome, and his ‘proof’ is a bit underwhelming. It certainly is not provable from Scripture.
And at the Temple, it was Jesus who was extraordinary.
Yes. As a general proposition the plain natural reading of the Scripture is also the Catholic reading. It requires a mariophobic mind of a 20c Protestant to invent the doctrine of sinfulness or non-virginity of Mary when there is zero scriptural support for it, then demand prooftexts from Catholics.