Did Virgin Mary bear other siblings aside from Jesus, as non-Catholics claim? Why it is hard for others to accept that the Blessed Mother had only one Child even if it's already crystal-clear in the Bible?

Non-catholics assert that Jesus had brothers and that Virgin Mary had conceived other siblings aside from our Lord, drawing their references from:

Mark 6:2-3 When the sabbath came he began to teach in the synagogue, and many who heard Him were astonished. They said, "Where did this Man get all this? What kind of wisdom has been given him? What mighty deeds are wrought by his hands! Is He not the carpenter, the son of Mary, and the brother of James and Joses and Judas and Simon? And are not His sisters here with us?" And they took offense at him.

Matthew 13:54-56 He came to his native place and taught the people in their synagogue. They were astonished 28 and said, "Where did this man get such wisdom and mighty deeds? Is he not the carpenter's son? Is not his mother named Mary and his brothers James, Joseph, Simon, and Judas? Are not his sisters all with us? Where did this man get all this?"

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What does the word "brother" in the stipulations of the Bible wish to imply?

In Semitic usage, the terms "brother," "sister" are applied not only to children of the same parents, but to nephews, nieces, cousins, half-brothers, and half-sisters, or even a spiritual brother.

In Genesis 13:8; 14:12 for example, the word brother in here is being used to describe the relationship between Abraham and Lot, who were not biological brothers but uncle and nephew.

Brought by the Bible’s broad semantic (the study of the meaning of linguistic expressions) range of “brother,” it can be taken that although St. Paul writes: “Jesus​ appeared to more than five hundred…brothers at the same time” (1 Corinthians. 15:6), we need not conclude from this verse that Mary gave birth to more than 500 children.

What about the alleged other Children of Mary?

These “brothers” in the above-mentioned verses are never once called the children of Mary, although Jesus Himself is (John 2:1; Acts 1:14). 

Are there other women named Mary aside from Virgin Mary

James and Joseph (also called Joses), who are called Jesus’ “brothers” (Mark 6:3) are indeed the children of Mary but not Mary, the mother of Jesus.

After St. Matthew’s account of the crucifixion and death of Jesus, he accounted:

There were many women there, looking on from a distance, who had followed Jesus from Galilee, ministering to Him. Among them were Mary Magdalene and Mary the mother of James and Joseph, and the mother of the sons of Zebedee (Matt. 27:56; Mark 15:40).