Where does it say in scripture that Mary was without sin for the rest of her life after having Jesus?
This happens to be the Holy Tradition of the Church, consistent with the description of her as full of grace (Luke 1:28), but no, it is not said in the scripture directly as such. Most dogmas of the Church are not spelled out in the Holy Scripture either.
The Pope is the Vicar of Christ on earth. He has been given the authority by Jesus; “I will give you the keys of the kingdom of heaven; whatever you bind on earth will be bound in heaven, and whatever you loose on earth will be loosed in heaven.” Matthew 16:19. In this verse, Jesus is speaking directly to the apostle Peter. In otherwords the Pope can interpret the Bible anyway he feels like interpreting it. And it’s also a Catholic document. No protestants had a hand in compiling it. But they sure love to criticize the people that did.