J.B.
I would just flat out ask her before going there if any of her kids are sick and if yes, with what. Ask her before they come to your house as well, and explain to her that since you work outside the home, when your child gets sick it really puts you and your husband in a bad situation due to missed work. I am not happy with people who spread their kids' sickness around without care!
About the behavior -there's not a lot you can say to her, BUT if she leaves all the care up to you when you're all around one another, discipline them as you see fit (I would stop short of popping or spanking if you do that). If there's a problem, tell her to please be more involved with them because they're doing______(insert whatever).
I will say one thing in her defense. Cancer, chemo, radiation and major surgery wear people out like nothing else -including pregnancy, childbirth and the little sleep one gets with a newborn. Everyone I've ever known who went through or is going through chemo or radiation has said they've never known such a bone-tired feeling in their lives (and many of them have lots of kids they've raised). It's evidently the kind of tired where not only do you want to lie down in the floor and sleep anywhere, but you will do it! She shouldn't ignore her kids' behavior or drag them around sick, but she probably is EXTREMELY exhausted all of the time and she does not feel well.