The Will does not bar Prince Gustav to marry Ms Carina Axelsson, they can go to the municipal registrar right now. So they are not infringed in any right at all.
Carina is not Standesgemäß, a requirement set by the deceased. The question pops up whether it is unlawful when a deceased owner of a fortune has set a condition that the benefactors of his Will must contract a marriage which is in line with the longstanding tradition of his House?
What would we say when Prince Gustav is still unmarried but a younger brother had married indeed, with a lady meeting every requirement as set down by his grandfather in his Will? The deceased has left an imposing castle, a gigantic estate (the prince is one of Germany's largest private landowners) and a vast portfolio with real estate and investments. You can see why a younger brother would have challenged Gustav, like also his cousins from the Sayn-Wittgenstein-Hohenstein branch can do. It is not exactly about a fancy title, it is about real wealth, then we know enough...