Why don't viruses have a little respect for schedules?

Some kids from playgroup have come down with the chicken pox. While I am in no way opposed to the girls getting the pox (I actually get a giggle out of saying, “WE HAVE THE POX!”) the timing is not so good. We’re planning a trip up North to visit Mark’s family and attend his 20th High School reunion next week and if my girls are infected, they should be developing the angry red blotches right around the time we’re scheduled to leave. While driving around the state with two sick kids is enough to put us off, the really hard part is that those people we’re visiting make the issue even graver. Mark’s mom suffered a pretty horrible car accident a few years back and had to have her spleen removed as a result. This means she is immunocomprised and has to be very careful with her health. She has other health issues as well, so there is no way in hell we’re bringing our children close to her if they are sick. We’re also planning to visit his grandfather, a man who is 91. And we’re just not that stupid.

So now I’m trying to figure out what to do. Leave for our trip as long as their not showing signs? But then what if they get sick while there? Cancel the whole trip? Send Mark to the reunion without me and take care of two (potentially sick) kids by myself? Thoughts? Advice?