Abby, can you get any kind of guideline from your sister at all? I know that as a mom, if I were going to someone's home, I would LOVE it if they asked me what would be good suggestions to have for my child to eat. I know that I personally would let mine have a balance between things that would be considered treats and healthy things, and I'd give a lot of leeway and flexibility. It could still be a surprise for the kids, and you would feel great knowing that they enjoyed what they had and that everyone (including their mom) was happy.
If you don't want to go that route and you think your sister might balk at cookies, you could always find a recipe for some healthier ones, like oatmeal raisin with flax meal and bran, or carrot cake, you get the idea.
Enjoy the visit!
And now that you don't have to be perfect, you can be good.