OK, bear with me as I post scriptual references to virginity as being the more perfect state. The implication being that if it is for the rest of us, it was most certainly for the Blessed Mother.
And aren't there ambiguous scriptural reference(s) to siblings of Jesus, too?
Aramaic uses the eqivalent of "kin" and apllies it to cousins as well as siblings. These are the references you mean, as to the Apsotle St. James the Lesser, the cousin of Christ, who later became the first bishop of Jerusalem.
As for Catholic practice, the ancient prayer of the Confiteor" "I confess, to Almighty God, to blessed Mary ever virgin, to blessed Michael the archangel,...."
Don't you think, even as just a practical matter, that subsequent marital relations would cause the supernatural origin of Christ to be questioned?
As far at the Catholic prayer goes, they represent church beliefs. Like I said, I don't doubt that church organizations came adhere to beliefs (some that have dubious scriptural backing).
So for me, I still don't see any explict scriptural reference to Mary remaining a lifelong virgin, and extrapolating other references to that situation is dubious at best - there is explict scriptural reference to being fruitful and multiplying, as well as establishing a carnal relationship with a spouse - to me, that's not notably corrupting.
Jeremias xxxi, 22: How long wilt thou be dissolute in deliciousness, O wandering daughter? [i.e. Israel] for the Lord hath created a new thing upon the earth: A WOMAN SHALL COMPASS A MAN.
Matthew 1, 18-25:
18 Now the generation of Christ was in this wise. When as his mother Mary was espoused to Joseph, before they came together, she was found with child, of the Holy Ghost.
19 Whereupon Joseph her husband, being a just man, and not willing publicly to expose her, was minded to put her away privately.
20 But while he thought on these things, behold the Angel of the Lord appeared to him in his sleep, saying: Joseph, son of David, fear not to take unto thee Mary thy wife, for that which is conceived in her, is of the Holy Ghost.
21 And she shall bring forth a son: and thou shalt call his name Jesus. For he shall save his people from their sins.
22 Now all this was done that it might be fulfilled which the Lord spoke by the prophet, saying:
23 Behold a virgin shall be with child, and bring forth a son, and they shall call his name Emmanuel, which being interpreted is, God with us.
24 And Joseph rising up from sleep, did as the angel of the Lord had commanded him, and took unto him his wife.
25 And he knew her not till she brought forth her first born son: and he called his name Jesus.
{Nothing implied about aftwerwards by this in the least. Virginity references next.}