When I told them my wishes, they attacked me with a barrage of negatives about cremation, such as, "You won't go to heaven," "You won't see your deceased mother or grandson in heaven,"

This is so alien to every branch of Jewish theology I have ever been taught that I have difficulty believing this letter is real. (Or, I mean, maybe it's real and the kids are actually evangelical Christian, but the LW changed it to Jewish to anonymize without thinking through how little sense that would make? I don't know.)