Scannie here, wanted to drop some notes I forgot to add:
When Joy says she'll "write [Julia] a reading prayer" it is in reference to a part of the Korean funeral tradition to offer a prayer to the dead.
Second, whilst in past chapters I had Joy refer to Julia as sister or just by name, as this is a canonically Asian-American family I thought it'd be more intimate if Joy called Julia "unnie" aka older sister. The series uses a mix on English and Korean in-universe so same goes for Joy's letter being in Korean and not English.