Just tell her you don't want them eating McDonalds, and provide meals for when she babysits. If she still ignores you, then you'll have to decide whether you want her babysitting anymore.
I was living with my mum when I had my son. She paid no attention to what I wanted whatsoever. She ignored absolutely everything I said, disregarded the child rearing methods I wanted to use, and refused to hand him over when I asked her to. (even when carrying him near a slippery riverbank, and she can't swim.) After THAT incident, we moved out.
If it's important to you, make sure she knows your feelings about the food in a way that she can have no doubt its not negotiable. Then see what happens. Sometimes clear and blunt is all that works. Sometimes even that doesn't work.
Good luck with it all anyway!
Suzy.