Illinois child protection laws?

I would call CPS. It sounds like those children are in an unsafe environment. The house sounds dirty. You said there is a smell, particularly of something dead; that could be a hazard for children because of what they are breathing in (animal feces, animal urine, or carcass). There is probably mold in the house, because of the sink full of dirty dishes. That also means there could be a pest infestation because of food left lying around, or dead animals. Also, the children could get physically injured due to the uncleanliness of the house. If there are clothes everywhere, and there isn't a place to sit, I'm assuming that the entire house is a mess and that the children could injure themselves. I do not think there is anything CPS could do about the food; if the children are being fed (whether it is a nice meal or crappy Mcdonalds every night), they are still being fed, but this house could be severely affecting their health. I would call and have someone come look at this, because it sounds very unacceptable for children to live in, especially young children. You could take your sister to court and ask for custody of the children until your sister gets her house clean (free of dirty dishes, mold, bugs, feces, dead animals, etc).