The concept of Mary’s perpetual virginity is conveniently explained in The Protoevangelium of James since James is viewed as an older step-brother brother of Jesus being a child of Joseph and his first wife, prior to his marriage to Mary. read more
Now the birth of Jesus Christ was on this wise: When as his mother Mary was espoused to Joseph, before they came together, she was found with child of the Holy Ghost.
And knew her not till she had brought forth her firstborn son: and he called his name JESUS.
Then said Mary unto the angel, How shall this be, seeing I know not a man?