I agree with the others: you're going to have to tell her that you won't watch them if they're sick. Granted, they say that people are most contagious before they show symptoms...so there really isn't a way to eliminate ALL risk of infection.
But to me it's logical: If there's fever, diarrhea, vomiting, or the first day or two of sniffles, coughs, etc...you cannot risk your health by babysitting!
My heart goes out to you, though, about not wanting to seem germ-phobic. Lots of people today are that way, without a good reason to be.
One of my dear friends, Pam, is dear to me in every way except that she just doesn't "get" the whole germ thing. She's really pressing me to do a child-care swap with her when she goes back to school in the fall. I'd love to help her out, and before my diagnosis, I would have. But her little one is three, all three of her older kids are in elementary/middle school, and the little one stays sick. My two boys are homeschooled, and though they get together with other h.s'd kids a couple of times a week, they aren't in big groups of other children, exposed to things all day long. It's very rare that either of them get sick anymore. But I fear that if I watch her little boy we'll be back to catching everything he and his brothers bring home.
I was very laid-back about the germ issue the first year or so after my diagnosis. Then, a simple respiratory infection ended up causing myositis (muscle inflammation). It took months of physical therapy to get over the pain and to regain full use of my shoulder and hip joints. In addition to homeschooling my young sons, I'm a freelance writer, and I just can't take the chance of adding more infection risk to my life. I have too much to do.
Ordinary people get a cold and they're down for a maybe a day, then sniffle for a week. We get a cold, and we could be the same...OR, we could be in bed for weeks and have it turn into all sorts of complications. It's just not worth it.
I think it's admirable of you to take care of these girls. But you have to look out for yourself, too. What'll she do if you catch something from them, and then you aren't capable of watching them at all for weeks or months until you're better?
Sorry that got long, but I have strong feelings on the subject!