Well, it's very obvious that your mother is being absolutely ridiculous and you are completely in the right for wanting to take care of your daughter's health, and I applaud you for that. However, I do not think that your solution is the right one.
Completely breaking off the relation between your daughter and her grandparents when they obviously love each other so much is way too drastic I think. It seems especially unfair towards your father, who, as I read your posts, is not directly responsible for the situation (except for not telling his wife to back off of course). Its clear that you and you're mother don't get along, but have you ever considered that instead of just dropping off your daughter for a whole weekend and handing over all responsibility you can also visit your grandparents with the entire family for a day every few weeks or so, so you can supervise yourself what she's eating?
I disagree, being a parent myself. I have experienced inlaws and relatives spoiling my children with not only unhealthy treats, but excess material posessions.
I had to stand my ground and restrict visits until the behavior stopped, and yes, it made me the bad guy. Oh well.
Poor eating habits begin in childhood, and children need to be taught what is and ins't proper nutrition. Things like poptarts, pancakes, and breakfast pastries are not healthy, and actually cause blood sugar spikes, then lows, which propels the individual to desire more carbohydrates. A good healthy breakfast is not cold cereal with milk. It is whole grain toast with peanut butter, (or poached /soflt boiled eggs as a protein). Fruit juice should instead be replaced by whole fruit and a glass of water. Juice is marketed as healthy, but in reality, it is one of the worst things a parent can give a child. Sugar is sugar, whether it comes from fruit (whole fruit contains fiber) vegetables, bee vomit, or grasses. Juice is liquid fructose, a sugar. My kids never had juice growing up. To get a child to eat healthy, the parent has to set and example, and to keep the child feeling a part of things, they should be allowed to choose fruits and vegetalbles for their weekly snacking and meals. This can be done on grocery trips, and is a great way to incorporate real world lessons in simple mathematics such as addition, subtraction, multiplication, and division.
That said, my fifteen year old son prefers pasta and bolognese sauce for breakfast, over anything else. He's healthy, runs cross country, and turned out mostly fine. Now If I can just get him past the teenage years, life at home will be peaceful.