Oh, come on. Author, you have no other ideas than this. Sorry, just forced and stupid. Selling what was already sold.
See also ch 2 where it is said: "... under house arrest at the residence at our duchy." Yes, it belongs to them.
She is the daughter of a duke. Duke is second in rank. If they would go to court against a Real Estate nobody ...
Also, keeping your whole money you need to maintain a complete mansion in some idiotic places. You would see this only when there not much money in the house.