Wow, thanks for pointing out the Arco-Zinneberg ruby parure. That parure is so rarely seen, that I only knew the pics from the pre-wedding of Guillaume and Stéphanie. So this lady would be Countess Maria Theresia, née Countess of Preysing-Lichtenegg-Moos? Or her senior sister-in-law, Countess Eugenie?
Based on the pictures I have seen in her senior years I would say that it is Countess Maria Theresia...