Telling A Pregnant Woman They're Sick

So, my brother in law, and my twin sister tend to ask myself, and my husband to babysit a lot, regardless of the fact that I'm 8 months pregnant. I normally don't mind at all but my oldest son is 6, so there isn't a lot of chasing him around that I have to do. Their kids are 7, 2, and 1, so babysitting for them can be quite a handful. The other day though, they asked us to babysit without telling us that their kids are sick. Their 2 year old has diahrrea, and the 1 year old may have the flu bcuz she has basically all the symptoms. Or at least a stomach virus of some kind. I was furious that I wasn't told before agreeing to babysit them that the smaller ones were sick since they can sometimes require diaper changes, and I don't want myself, or my 6 year old getting sick AT ALL. And the brother in law is a stay at home dad who mostly only asks me to watch them so that he can go to the store or something, which I understand, but I told them next time if they're not feeling well, then I want to be told that. I can't risk getting sick, and going into labor and I can't hold my own newborn because of you all. And I don't want them coming to see me in the hopsital if anyone at home is sick bcuz they might be carrying the germs with them. I'm a bit of a clean freak but do you think I'm overreacting.??