Of course Christ was conceived in Mary’s womb solely by the Holy Spirit. But it is also an article of faith that Mary remained a virgin throughout the rest of her life. The earliest written reference is from 125AD, in living memory, and many of the Church Fathers from the 3rd and 4th centuries affirm that Mary’s perpetual virginity was always held by the Church: St Athanasius, St Jerome, St Ambrose, St Augustine, St Cyril.
Sometimes the Gospels refer to the ‘brothers and sisters’ of Jesus. However, it does not say these are children of Mary. Semitic languages like Aramaic did not distinguish between blood brothers/sisters and cousins or other near kin. In fact there was no word for ‘cousin’ at all. Abraham’s nephew Lot was called his ‘brother’ (Genesis 14:14). Likewise in the Greek of the Gospel, brother/sister (adelphos/adelpha) also includes cousins and others. So John the Baptist was a ‘brother’ of Jesus even though he was the son of Elizabeth and Zechariah. We also hear that Moses visited his ‘brothers’, meaning his descendents (Acts 7:23).
Finally, on the Cross Christ gave Mary and John to each other. The Gospel says John made a place for her in his home (John 19:27). This only makes sense if Jesus was Mary’s only family.
This question is important. Every doctrine about Mary tells us something about Christ or ourselves and the Church. Mary’s perpetual virginity demonstrates her total dedication to Jesus. And Mary is the model of the Church: both mother of the faithful and ‘pure bride to her one husband’ (2 Corinthians 11:2).