The hairstyle she wears in the short story art matches the girl in the preview from the Live Letter, too. I saw it was a book and IMMEDIATELY clocked that was Y'shtola's baby sister.

The entire gimmick of the short story was that, despite the distance and lack of relationships most of the sisters have with each-other or their father, Y'rhul left each a book that was extremely in-line with his daughter's individual interests, and all of them were insanely valuable and/or rare. He collected odd tomes as a hobby. (He left Y'shtola a super rare and very likely forbidden Mhachi text on void magic. Man was plugged TF in. The older sister on the forum was very not happy about that choice.)

There was no brother. Once again the lore seems to go back and forth on if boys are just rarely born or not.