You sound like a very sweet and concerned sister. Children at 4 years old tend to be not only picky eaters, but also follow the example of others. For example, if you only eat good foods all day long (no chips, popsicles, or any other junk food), then she will eventually get the idea that eating good food is better for her. Some of this is going to have to start with your parents not keeping these foods in the house in the first place, no matter what your sister says or does in the store. If she throws tantrums in the store, how about your parents getting a sitter to watch her while shopping for food so your sister won't know what is being bought?
As for her tantrums when she doesn't get her way: she does this because she has gotten her way every time she has thrown a tantrum. It is actually up to your parents to stop this. Everyone needs to teach her it is not ok to hit anyone at any time for any reason and that she cannot have her way all the time. How can this be done? It is going to be hard to do, and going to take quite awhile (it will get harder before it gets easier) since she is already 4 years old and already gotten away with it for so long. The best way to handle a tantrum is to first off make sure that she is put into one certain area that is away from everyone, such as on the stairs, on a chair facing a wall, or something like that. She cannot be where she can play with anything or talk to anyone. She cannot be allowed to talk to anyone or say anything. The longer she cries or talks, the longer she stays there. Since she is 4, she can stay there for 4 minutes once she is quiet to think about what she is doing. Then she can come out to talk about the problem. This is to be done any time she is hitting or throwing a tantrum and she is to be put there by your parents, not you. She is not to see you as a parent.
I am wondering why this little girl is even awake at midnight in the first place! In my book, she--and you--should be in bed and asleep by no later than 8 p.m., even during the summer! However, that is up to your parents. A 4 year old does need at least 12 hours of sleep a night and a 11 year old needs close to that amount.
Sweetie, all of these things are up to your parents to enforce and take care of. Please show what I have written to them if any of it makes sense to you or helps you. It sounds as if you truly care about your sister and want to help her. If that is so, then please get some help with this as you cannot do this alone. It takes more than one person to do what you are wanting to do. Good luck to you and your family!