I noticed that they have not be seen for a while but could it be that until the court case is resolved they are laying low as a couple? Wasn't one of the arguements against Gustav the fact that he is in a relationship with someone he could not marry according to his grandfather's will? I thought that came up somewhere.
The judges look at the legal set up of the House properties not to the relationshipa of the individuals. Not even the conditions are at stake (the Chef of the House should marry a fellow protestant noble lady) but the possible workings of the Will as a Fideïcommis (binding the hands of a heir). In many countries outside the Anglosaxon sphere such a Fideïcommis is not allowed.
An example:
A = grandfather
B = father
C = son
A leaves 100.000 Euro to B, on condition he passes this amount to C (but can use the rent on the 100.000 Euro) . This is letting B inherit the 100.000 from A indeed but prohibiting him to use it as he, on his turn, has to pass it to C.
That is a real Fideïcommis: an inheritance in three stages, surpassing generations.
The complicated legal question however is if there is an unlawful fideïcommis here. The Fürst (Prince Gustav) has the complete usufruct of his enormous inheritance. He is not limited like heir B in the example I gave.
The only point is that there is a desire to keep the properties from generation to generation in the hands of the Fürst, respecting House Laws (the Fürst has to maintain the standing and prestige of the House by conducting a marriage within his rank and keep the House Protestant).
Does that affect Prince Gustav's human right to marry anyone he likes? Not at all. He can marry a Hindu bride from New Delhi tomorrow, if he wishes so. No problem. But then he breaks the House Laws and another Berleburg whom did marry according rank indeed will knock at the door and point to the legal arrangements.
The question is if it is illegal to have a condition attached to a Will. We have already seen that it is illegal to limit heir B for the benefit of heir C. But that is not the case here. Is it - for an example- allowed to have a Will which says that money must be kept in trust until the age of 25? Is it allowed to let your son inherit your house on condition that your wife/his mother, may live there for the rest of her life? These conditions are not usual.
The question is: is it reasonble that the Head of an aristocratic and protestant House has the usufruct of the House fortune under condition of keeping said House aristocratic and protestant? Note that the knife has two edges: exactly because of these very same House Laws, Gustav got an imposant ancestral estate, studded with unbelievable artworks, is one of Germany's biggest private landowners and enjoys great wealth.
It is a very complicated legal minefield with pitfalls. Gustav of course also wants to protect the centuries old estate and does not want all protection go, seeing it fragmented over dozens of heirs, subjected to hefty taxes and resulting in loosing the economic model of castle + lands + enterprises together maintaining the ancestral estate.