I think you need to talk to your son and point out that you asked not to have your grandchild if she was unwell because your immune system is compromised, which of course means that any and all infections are potentially more dangerous for you than for other people.
Please, please, stay right out of any discussion about your DIL or how the young couple are spending money, unless and until your son comes right out, off his own bat and asks for advice.
Likewise whether or not they have more children is entirely their own concern. I do appreciate that you are worried that another baby would be a very bad thing when your DIL is so ill, but again, it is entirely up to them.
IMO you will be justified, if they do have another child ,in pointing out that you are happy to look after one grandchild at a time, but not both of them at once, if you feel that would be too tiring for you.
It sounds to me as if your DIL certainly cannot be expected to do her own cleaning. I have tried bending with a violent tummy upset and, I assure you, it is not a wise thing to do. With her condition, house-cleaning is out of the question, and cooking might well pose a health hazard to those who ate anything she cooked.
If you get tired of hearing your son moan about money, ask him how serious the problem is, and offer to help him make out a budget, something many young people have no idea how to do, but please, don't criticise his wife, I'm sure you remember, as I do, how mad it made you when your mother criticised your husband.