My point is that Matthew is not saying anything about the marital relations of St. Joseph and St. Mary in that verse, neither by way of knowledge of conjecture. Again, you set out to prove a contradiction to a Catholic doctrine and the verse does not contain a contradiction. Remember, the same word "till" is used in very many ways besides "until", as I showed you from scripture.
But you said Matthew didn't know. How could he have spent so much time with Jesus and not known whether or not Jesus had biological half-siblings? He must have known. That strengthens the "until" view.