Well, there's a hierarchy between them. For instance some of them, but not all, are "pairs de France", which gave them the right to sit in the Parliament, like Roman patricians had the right to sit in the Senate. If I remember well the rank of archduke was created by an Austrian emperor, and it didn't spread to France, since relationships between the kingdom of France and the Empire never were very good (after the reign of Carolus Magnus). It must be a similar story for the rank of grand duke.Interesting that in France there are only Dukes!
It might be a reason why it's a huge problem for her to fall in love with a commoner . But sometimes, if you kiss a toad, a prince suddenly appears.As for the script, I think "van" is a writing mistake if the majority has Bella with "von Halen". As mentioned before, it would even be an interesting point for Bella as a character if she is from old nobility, no matter how high in rank.
I also thought that "van" was a typo. Ocean might have have been dreaming of the comet.