What do you call people who are related to the king abroad?

There is no clear title for the king's friends, but generally they are nobles, "princes, sons" and other titles.

The master of a king is called a teacher or mentor, and there is no such thing as a national teacher in European countries.

The king's brother is called a prince.

If the king's sister is married, call her by her husband's title; if not, call her princess.

The successor is called the Crown Prince.

In short, everyone is called a member of the royal family, and sometimes he can become a member of the royal family. Although the word has some oriental flavor, it can also be called that.