This teaching is rubbish. Mary had other children. What Catholic doesn’t believe in a large family?
The following is from the Gospel of Matthew, an Apostle who knew Jesus and His family background intimately, even without the inspiration of the Holy Spirit: “Is not this the carpenter's son? Is not his mother called Mary? And his brethren, James, and Joses, and Simon, and Judas. And his sisters, are they not all with us? Whence then hath this man all these things? (Matt 13:55, 56) These comments were made by people who knew Joseph and Mary and their family, for the Scripture tells us in the preceding verse: “And when he (Jesus) was come into his own country, he taught them in their synagogue, insomuch that they were astonished, and said, Whence hath this man this wisdom, and these mighty works? (Matt 13:54) It bears repeating that these comments were made by people who most certainly knew the difference between blood brothers and sisters and mere cousins or kinfolk. We find a second report of this incident in the Gospel of Mark.
“Is not this the carpenter, the son of Mary, the brother of James, and Joses, and of Juda, and Simon? and are not his sisters here with us? And they were offended at him.” (Mark 6:3) As additional assurance that those who were making these comments were very well acquainted with our Lord’s earthly family, we read: “But Jesus, said unto them, A prophet is not without honour, but inhis own country, and among his own kin, and in his own house. And he could there do no mighty work, save that he laid his hands upon a few sick folk, and healed them.” (Mark 6:4, 5)
In Greek, the language of the New Testament, the word for brother/ brethren is adelphos {ad-el-fos'}; for sisters, it’s adelphe {ad-el-fay'}. The word for cousin/kinfolk is suggenes {soong-ghen-ace'}. To think or believe that the inspired writers of Scripture were unfamiliar with these terms and therefore subject to misusing them, is to question the very integrity of the Holy Spirit who directed their efforts.