Caroline has been married three times. The first marriage was annulled before she married husband no. 2. They had 2 sons and a daughter. He died and a few years later she married husband no. 3, who has the title Prince of Hanover and they have a daughter. Her sons would be legitimate heirs. Not sure about Stephanie, some of her children may have been born in wedlock or not. She may have a son, I pretty sure she has more than one daughter. Too lazy to google.
A great scenario for an Agatha Christie novel, in other words.