she's not an actual princess but daughters of duke's are referred to as princess. Though they are not put on the same level as an actual princess (aka a member of the imperial/royal family). sometimes they're referred to as lady *insert last name here* or princess.
since dukes are usually relatives to the royal/imperial family. typically brothers of the king/emperor would be given duke titles and in the event of the death of the royal/imperial family (with no heirs) the duke can then take claim to the throne.
because of this they are sometimes called prince or princess since they technically are still royalty, they are just branch families of royalty.
Maybe I forgot why but I’m confused that they call her princess when her dad is a duke?