My question is do you want her there? And I don't mean just accepting she will show up invited or not, but do you want her there?
Either way, I would say something like:
"I'm sorry sis, but I was really embarrassed that you only bought a tiny potato salad to share. You have a family of 6, and it's really not fair for my Inlaws to have to cover food for your family."
and then if you don't want her there in future, add something like:
"My MIL and I were pretty upset and I feel you are taking advantage of their kindness in allowing you to come, so I'm sorry, but you will have to find other plans for Christmas from now on."
If you do want her there, or are going to accept she will just come anyway, then add something like:
"So if you plan on coming to my Inlaws family Christmas, I expect you to pull your weight and provide the same amount of food as everyone else does. I don't want to be embarrassed again next year and have my MIL upset again"
I understand where you are coming from. One of my sisters is a family of 6 and she used to give everyone else huge lists of things to bring, and do very little herself. When I was just one person and she gave me a list of about $100 worth of food and nibbles, I cracked the sh!ts and refused to do it. Another year she gave us a big list (it always had to be at her place as was convenient for her and Mum let her get away with it) so that my Gran wouldn't have to do anything, so I accepted that as Granny was getting on. Then Gran turned up with a full esky while sister had done nothing! I let rip at her that year as was so angry!
Which I why I love spending Christmas with my IL's. So much easier and nicer, and everyone does their fair share and it's just so much more relaxing.