How do I defend the Blessed Virgin Mary?

I have a new friend who tried to convince me that the Vigin Mary not only was the Mother of Jesus but also the mother of other sons! Therefore trying to force me to believe that Christ had biological brothers and therefore Mary was not a virgin! “What!” I exclaimed. But, heartbreakingly, I had no specific Scripture knowledge to debate her on this matter concerning my Catholic faith. Please help me understand with Scripture and how I should go about responding to her relentless efforts in trying to lead me away from my Roman Catholic faith.

Recommended reading:
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*]Mary and the Saints
*]Mary, Mother of the Son by Mark P. Shea
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