The perpetual virginity of the Blessed Mother is one of those things carved in stone. It is unalterable fact. If you wish, you may adhere to a theory that the “brothers and sisters” of Jesus are half-siblings, from a (hypothetical) previous marriage of Joseph. It’s not a well-founded theory however, in view of Biblical testimony to the names of both parents of the “brothers” of Jesus.
BTW, did the learned Catholic priest who taught you these heresies teach you also that ancient Aramaic refers to remote male relatives as “brother” because it has no word for cousin?
Finally, you do not understand what the term Immaculate Conception means. You should. It’s important.
Interesting. By this same logic we could conclude that Abel was actually the cousin of Cain. Or that Aaron was actually the cousin of Moses. And so on.
From the Catechism and the encyclical, 135 Pius IX, Ineffabilis Deus (1854): DS 2803,
resources, including biblical and theological references:
www.ewtn.com/library/papaldoc/p9ineff.htm
491 Through the centuries the Church has become ever more aware that Mary, “full of grace” through God,134 was redeemed from the moment of her conception. That is what the dogma of the Immaculate Conception confesses, as Pope Pius IX proclaimed in 1854:
The most Blessed Virgin Mary was, from the first moment of her conception, by a singular grace and privilege of almighty God and by virtue of the merits of Jesus Christ, Savior of the human race, preserved immune from all stain of original sin.135