Like you said, you're not there 24/7 so whatever diet you decide to do, probably won't work unless you get the inlaws to agree to it. I have to agree with the person that said not to make a big deal out of it...the more you focus on her weight the more she's going to feel bad about herself etc. and it is a downward spiral. Especially if the ped is not concerned! I know it's hard to do!
If you get a chance, read the "Schwarzbein Principle." It is written by a medical doctor who specializes in hormones. You may not want to follow her specific diet in total, but the overall principle is very very sound.
My 5 yo is actually pretty good about not eating sweets without my permission, because I've explained to her about health. I've also gotten the doctor to tell her that it is not healthy to eat sugar, and if she eats it at all it should be very minimal. Mind you, she wants to eat them, but she at least will ask me first. I would tell your daughter that she needs to ask YOU, and not the grandparents, and if you find out she's eating without asking, then she will have some consequence (like no sweets at ALL for a day or something). It might sound crazy, but like I said, it works for my daughter but she is pretty obedient anyway.
Do your inlaws babysit your daughter? What would you do if you weren't living with them, as far as who your daughter was spending time with? I would try to limit the time she was with them without you around, as much as you can. Also, are they the ones buying the "junk?" Maybe you could see if they'd be willing to keep it out of the kitchen at least. I know you feel they won't listen, but maybe if you explain the health side of it instead of having a shouting match, they'll at least start to see your way. Print out articles from the internet about sugar, etc. Tell them they can choose to eat how they want to, but that you want your daughter to be the healthiest she can be and you hope they want what's best for her, too. Tell them you understand it's hard to say no or change what they've been doing, but that you want them to stick to your menu (put a list of okay snacks and meals on the refrigerator). Maybe they'll start to listen then?
Hope that helps!