Then what word should they use?
Literally “oppa”. To use as is, same with terms like “noona”. The connotation isn’t the same when it’s literally translated into English because of the cultural difference
There’s certain words that shouldn’t be translated because they loose their intent. It’s similar to the word Daddy or Master. It means something totally different depending on the context they can be innocent words or imply a bdsm dynamic or be incestuous. Oppa is similar. We’re now trained to recognize that it has many meanings besides just brother.
Please, don’t translate the word “oppa” literally. The princess is out here calling her cousin her “brother” while we know she means to marry him…