I can relate a little to your worries and situation. I have a five-year-old son (Sean) who has CF. Around Christmas time, we thought our nephew might have CF, as well. It is a long story (but happily it had a happy ending - they ultimately decided that he was only a carrier of CF with a severe milk alergy). However, during that time, one of our worries was what to do with the two boys. I actually asked around on a different web site about other's opinions. I heard many stories about cousins and siblings with CF who all played on a daily basis with each other. Ultimately, we decided that it would do more harm to keep the two cousins apart and we decided to use a lot of common sense and caution with the two when they were together. We decided that we wouldn't get together if either of the boys were sick or could "cross-contaminate" each other and that we would be careful about shared toys, etc. Because of their ages (5 months and 4) at the time, we didn't see masks as a solution. We also knew that we wouldn't be able to effectively enforce the arms-length rule. We decided that we would be careful about germs - we used lots of lysol and hand washing during that Christmas visit.I hope you also have a "happy ending" and that your nephew is healthy and you do not have to worry about this issue. However, if he does end of having CF, I think it is a decision that needs to be made between you, your husband, your sister-in-law, and your brother-in-law. This is something that all involved parties need to be comfortable with. I hope this helps. I wish you and your family lots of luck. Please let us know how his sweat test turns out. Katrina (mother of 3 - Sean (5) w/cf, Carolyn (3) w/o cf, and Allison (1) w/o cf