The Benelux Royals Message Board: Luxembourg styles

As I understand it, only the sovereign and heir were Royal Highnesses, with cadet members being Grand Ducal Highnesses. But the two eldest daughters of Grand Duke Guillaume IV succeeded to the throne as reigning grand duchesses, with two other daughters marrying into the royal rank. However, both husbands came from non-reigning dynasties (Saxony and Bavaria). Nevertheless, was the Royal Highness style legal everywhere -- in particular, recognized in Luxembourg?

It has occurred to me that a similar thing happened later on, when Charlotte married her first cousin, Prince Felix of Bourbon-Parma -- who, despite being a member of a long-deposed dynasty, was a Royal Highness as a male-line descendant of King Louis XIV of France (itself a long-abolished monarchy). Accordingly, their own male-line descendants have been elevated to that style.

Nevertheless, isn't the style of dubious validity, in light of the fact that it comes from the non-reigning royal house of Bourbon? Yet, it is being applied to members of a reigning grand duchy ...