Consider the Eastern church.
They developed with their own theological traditions and expressions, and yet they also agree (in substance) with the Latin-expressed views of the Immaculate Conception and Assumption.
Easterners express the Theotokos as “all-holy.” Whereas the fine details of the Immaculate Conception are not expressed in Eastern theology – mainly because the related concepts were not expressed in the same way in West and East (like Original Sin), the Orthodox DO regard the assumption to be a REAL event.
But if the assumption (dormition) is a real event, then that strongly suggests Mary’s sinlessness — for the wages of sin is death.
So, in other words, the fact that the East has the substance of the same beliefs – especially Mary’s dormition (which suggests her sinlessness) – then don’t worry about it being a Roman Catholic invention.