Baby keeps getting sick from cousins

This might be a little long and mostly venting, but also asking for some advice. So, a little back story...I've always been a bit of a germaphobe and having kids made it worse. I keep my kids home if they coughing/sneezing, 24 hours after fever, etc. I know kids spread germs and get sick so I don't expect them to be healthy all the time, but we try not to spread our germs.

So now on to the issue. I have an 8 month old and a 4.5 year old, my brother and his wife have an 18 month old and a 5 year old. They live a couple of hours away so we don't see them a ton, but often enough. My 8 month old has been sick immediately following visits from the 3 of the 4 times she has been sick (first time she was just a few weeks old). They never tell us ahead of time if their kids are or have been sick so I don't know until we see them. I was already getting annoyed when daughter got sick right after Thanksgiving, but then they decided to show up for Christmas barely 24 hours after having a stomach bug.

So my 4 year old missed Christmas because he got sick on Christmas eve and now my 8 month old has it. I guess my question is, what can I say or do to encourage them to stay home (or at the very least give some forewarning) if their kids are sick? I want my kids to be able to play with their cousins, but not if it means getting sick every time