why do Catholics believe that Mary remained ever-virgin despite the New Testament speaks of the brothers and sisters of Jesus?
Nowhere in the New Testament does it ever recognize the “brothers and sisters” of Jesus as the sons or daughters of Mary, his mother. According to Old Testament usage, close relatives could also be referred to as “brothers or sisters.” This is made clear in paragraph 500 in the Catechism of the Catholic Church.
When Matthew 1:25 explains to us that Joseph had no relations with Mary “until she bore a son,” this does not mean that Mary and Joseph had marital relations after Jesus’ birth. In 2 Samuel 6:23 we read how “Saul’s daughter Michal was childless until her death.” This does not mean she had children after her death!
The Fathers of the Church also realized evidence of Mary’s perpetual virginity in the New Testament. Mary’s reply to the angel, “How can this be, since I have no relations with a man?” (Lk 1:34) suggests a decision to remain a virgin. When our Lord entrusts the care of Mary to the beloved disciple (Jn 19:26-27), it implies that Mary had no other children to whom she could be entrusted.