It is so unfair, Gracey. It is so maddening. It is such a pain the a$$. But, it is the way it is.
You are not teaching your daughter that it is OK to be sneaky and hide things. Maybe she is learning that but it is not from you.
What are you teaching her? That when people do things that we feel are wrong, we should keep talking to them about it for the rest of our lives and try to change them? That we should put our own lives on hold while we try to fix them? That we should get so upset about their behavior that we can't enjoy the life that was given to us?
You can teach her, by example, that everyone is in charge of their own life. That people will do things we don't agree with and that is their right. It is not our place to try to change them. That she can make her own choice of whether she wants to eat a hamburger at a bus stop or eat with the family. If she's not hungry when the family eats, she isn't going to get anything to eat a 11:00 at night. She doesn't have to eat to make her grandmother happy or to make you happy. She needs to do what makes her happy.
If she doesn't learn that from someone, I would predict that she will be here on these boards when she grows up.
Hugs - L