Did the other relevant persons ask for the cremation of the grandmother? Was it her own request?
To make sure: The other people involved are also part of the extended family including both the grandmother and the people that ate her ashes, right?
Were the ashes sent to the receiving family due to a particular personal closeness they had to the grandmother, beyond simply being family (favorite son, eldest son, etc)? Is it due to the location they were at, such as the grandmother's childhood home? Were they sent there on request from the grandmother before her death?
Did the other people involved have a professional relationship with the family that received the ashes? Their family doctor? Physician? Or were they treated more as family? Friends? Acquaintances? Is this relevant?
Was the urn accompanied by a letter or other communication? That was misinterpreted? Were the ashes eaten (in part) due to a lack of information about them? Was a letter or other communication intended to be received first/at the same time, but didn't arrive properly? Was the family expecting something similar in appearance to arrive in the mail for other reasons? Was the sender of the urn clear to them?