Children have to be taught to care about other people's feelings. Obviously this little girl needs some training in politeness.
If I were in that position and could trust her parents I'd tell them what had happened and leave them to deal with it. You have done so and there is no more you can do except perhaps hope when she goes to school she will be taught empathy according to the capability of her emotional development.
This is especially difficult for you to be involved with considering your health problems. Please be assured your grand daughter's bad behaviour is not your responsibility. So you can emotionally distance yourself . You can do this by ignoring the naughty child or laughing at her tantrums.
A quote for those blessed with a spouse:

