Hmmm, statistically improbable, but we are dealing with this too. I feel very fortunate, as I have gotten to know the parents and we are going to work together, with our social worker, and the school's special services director and figure out the best plan. They all (twins with CF and Alyssa) start Kindergarten in the fall and our little neighborhood elementary only has one kindy classroom. Its likely that the school won't be able to accommodate this issue, so someone will have to use a different school, at least for Kindergarten. We'll deal with whatever it is, so long as we can maintain cross contamination procedures.
If the other family is open to it, I suggest getting to know them. It has made me much more comfortable knowing them and knowing we will work together to find what is best for our kids. Its a challenge, without question, but there is no need to freak out...stay calm, know what you need and what you don't, and start talking to the appropriate folks. your CF center social worker may be very helpful, especially if you don't know the family.