Tablet from Nineveh, letter from princess Serua-eterat, daughter of Esarhaddon and sister of Ashurbanipal, to Libbali-sharrat, wife of l Ashurbanipal, reprimending the future queen for not studying. 7th century BC. British Museum [2534×1423]

https://www.britishmuseum.org/collection/object/W_K-1619-b
In this letter, Šērūʾa-ēṭirat respectfully reprimands Libbāli-šarrat for not studying and also reminds her that though Libbāli-šarrat is to become the future queen, Šērūʾa-ēṭirat still outranks her as she is the king’s daughter (a title that would have been rendered as marat šarri, “daughter of the king”, in Akkadian) whilst Libbāli-šarrat is only the king’s daughter-in-law.

Translated into English, Šērūʾa-ēṭirat’s letter reads:

Word of the king’s daughter to Libbāli-šarrat.
Why don’t you write your tablet and do your homework? For if you don’t, they will say: “Is this the sister of Šērūʾa-ēṭirat, the eldest daughter of the Succession Palace of Aššur-etel-ilani-mukinni, the great king, mighty king, king of the world, king of Assyria?”
Yet you are only a daughter-in-law — the lady of the house of Ashurbanipal, the great crown prince designate of Esarhaddon, king of Assyria.