╮(￣▽￣)╭ Bad writing?
A daughter of a Duke is still considered a princess most of the time. This is because usually a Duke is the sibling of the king.
She's the daughter of a Grand Duke. A Grand Duke is the highest ranking nobility aside from the king or queen of a country, and in some instances they independently rule their own territories. A Grand Duke is referred to as "Your Grace" and their children will be a ducal prince or princess. While sometimes a grand duke or duchess will be related to the king or queen, that's not always the case. In cases where they aren't related to the monarch, a grand duke usually rules their own territories, making them a monarch in their own right.
For example, Prince Henry and Meghan Markle are the Duke and Duchess of Sussex (so Henry is technically a prince, but he is a Duke as well) and their children are referred to as Prince or Princess. Nobility is just confusing honestly.
ohh thats why but what if duke doesnt have successer. does he just adopt child and make it heir or closest relavities
If a duke doesn't have an heir, the title would go to a close relative such as a niece or nephew.
i dont understand shes daughter of grandduke but why is everyone calling her princess. samr goes for duchess