The quotes I gave on the previous thread not only bolster her claim, but illustrate that the Catholic church doesn't hold itself accountable to scripture aside from the verses above. The Catholic church can change whatever custom it sees fit, even if it isn't biblical.
I'm not sure you're that well-informed about the process? Let me assure you, any dogmatic declarations made by the Church are usually well-founded on Scriptural grounds.
If there's an instance you're aware of, I'll gladly discuss it with you.
A talking point would be the 'Four Marian Dogmas' declared by the Church.
The First: Mary is Theotokos – the Mother of God – declared at the Council of Ephesus (431). This is clearly argued from Scripture, notably Luke 1 and John 1.
The Second: The Assumption into heaven of Mary, body and soul. This was declared by the Church in the reign of Pope Pius XII in 1950.
There is no direct Scriptural support for this dogma whatsoever, but it is a truth of tradition of the Catholic, Orthodox, Lutheran and Anglican communions. It was the common belief dating back to the 5th century, and is actually attested in apocryphal literature from the 2nd. The stories differ, some place her passing at Jerusalem, others at Ephesus.
(There are also obvious mythological elements – in one, Thomas the Apostle (who was absent when the risen Christ appeared to the others in the 'upper room') was absent from the city when the body of Mary was laid in the tomb. He returned, and the tomb was opened for him to see, but was empty. The story goes on, but its clearly derivative.)
There are however Scriptural references which would seem to support the belief – Genesis 3:15 and Revelations 12:1.
The Third: The Immaculate Conception – by Pope Pius IX in 1854. This belief is that Mary, at her conception, was preserved immaculate from the stain of Original Sin. This rests on the angelic salutation in Luke 1: "Hail, full of grace, the Lord is with thee ... Fear not, Mary, for thou hast found grace with God." (v28, 30).
On my degree course we discussed this. Was Mary baptised, and by whom? Clearly, she was 'baptised' when she conceived by the power of the Holy Spirit. Was that immediately prior to the conception or, being known before the creation of the world that she would conceive, from the moment of her creation? The Church declared the latter.
(Note: a common erroneous belief is that 'The Immaculate Conception' means Virgin Birth. This is a mistake – it refers to her conception, not that of the child Jesus.)
The Fourth: The Perpetual Virginity of Mary, declared at the Synod of Milan in 389. Affirms the virginity of Mary before, during and after. Tricky one this: How do we know? And who would have 'the 'brass neck', as we say in the UK, to ask? And why?
There is a reference in an apocryphal gospel, but the proving of the hymen still intact seems just too invasive ...
Scripture talks of the family of Jesus, but not of the children of Mary ... so the tradition has always treated this as 'extended family'.
+++