LogFAQs > #923062033

If it were Beatrice, she wouldn't need a particular motive send the letters.

If it were a human, I don't have a clue.
The envelope was sent before the incident, which meant that the sender had to know for certain that Ange isn't going to Rokkenjima and would be alive to receive it. The Ushiomiya family wasn't told until they met up for the trip. Therefore, the only humans who have inside knowledge of Ange being sick, and could have sent the letter, was Kyrie's father, and by extension, the Sumadera family.