From the very early years of the Church, Mary was called not only Virgin, but Ever-Virgin. But the Bible mentions Jesus’ brothers, so how can this be?
The gate is shut
The Church Fathers often cite Ezekiel 44:1-2 as the prophecy of the Ever-Virginity of Mary:
And the Lord said to me, ‘This gate shall be shut; it shall not be opened, and no man shall enter by it, because the Lord God of Israel has entered by it; therefore it shall be shut.’
The gate mentioned is seen as a picture of Christ’s passage through the door of Mary’s womb. For two thousand years the Church as always taught that Mary and Joseph had no sexual union before or after the birth of Christ.
Jesus’ brothers and sisters
You might be thinking “But doesn’t the Bible mention the brothers and sisters of Christ? Who are they and where did they come from?” For one thing, they are never directly called the sons and daughters of Mary and Joseph. In the Semitic language “brother” and “sister” can mean kinsman, cousin or relative. For example, in the Old Testament Abraham and Lot are called brothers, even though Lot is Abraham’s nephew (Genesis 13:8; 14:14, 16).
The siblings of Jesus mentioned in the Bible are his half-brothers and -sisters, because Joseph was a widower. Jesus’ brothers and sisters are children of Joseph and his first wife.
Thus, Mary is known as the Ever-Virgin. The gate is shut, for the Lord God of Israel has entered by it.
Source
Excerpts taken from the booklet Facing up to Mary.