Taken at face value, Jesus had brothers and sisters from Mary; see Matthew 13:53-56. Jesus is in his hometown, and his townsfolk - the people he grew up with - questioned his teachings, saying he was the son of Mary, with brothers and sisters. In fact, his sisters were still with the townsfolk!
Now, if you want to assume that the Gospels meant cousins rather than actual brothers, you have to figure out why the word brother was actually used, especially in relation to Jesus' relation to Mary! Calling Jesus the son of Mary and that Mary and his brothers and sisters were together would seem to indicate a true brother/sister relationship, not cousins. All four Gospels (and Acts) agree, and all of them are quite explicit.
However there is no Biblical foundation for Mary never having another child that I know of; can you provide any supporting scripture that would indicate Mary didn't have more children? Not the Catechism, but actual scripture?
Seems this is a clear position where the Protestant and Orthodox churches are united in a clear position held since the founding of the Christianity and thus representing the true and accurate position of Christianity (especially since this position became Catholic dogma in the mid 1800s).
When a mixture of younger close relatives (e.g. half-brothers, cousins, blood brothers, adopted children) is described in one word, the Middle Easterners to this day use “brother” or “sister”. Specific words for “nephew” etc. exist, but it doesn’t mean they are used unless the precise relationship is the focus of the speech.
Even when a single person is referred to, and he is not a brother, the Scripture still uses “brother” at least in one occasion, when Abraham calls Lot his “brother” when in fact we know Lot was his nephew.
In a culture of large extended families sharing the household it is not unreasonable.
At least two of the supposed “brothers” are attributed to Mary Clopas and not to the Blessed Virgin in another gospel.
This does not prove that Mary never had other children. It just shows that the scripture prooftext about the “brothers” isn’t.
How we know that Mary had no other children? We infer it. First, theologically, as the totality of human fulfillment inherent in the choice of being the virginal Mother of God would seem to preclude other familial ambition. If God made a part of your body a holy-of-holies temple, would you fool with it? Second, if Jesus had blood relatives, why would St. John the Evangelist chosen as caretaker? Thirdly, it doesn’t seem that the purpose of the marriage of Mary to Joseph was to make babies anyway. When Archangel announces to Mary, she seems perplexed by the suggestion that she would become pregnant by anyone. “I know not man”, she replies. How many young brides do you know who marvel at the idea that they would soon give birth to a great man? She had never intended to be a mother, or to be a consummated wife.