I had to pipe in, too. My sister and I both have cf, and stressing over cross-contamination between siblings...well, I think it's kind of a worthless cause. As several people have said, these bugs can be picked up EVERYWHERE, not just from a sibling, and, also, when your back is turned they're sharing cups and toys and food, etc etc lol. Even as adults, my sister and I see each other several times a week, and we wouldn't have it any other way even if one of us did grow something before the other. We feel that our relationship is far more important than risking getting each other sick.
I think that if they're in the same house, just take the appropriate precautions to keep both as healthy as possible, but don't sweat the little things that come with being siblings. <img src="i/expressions/face-icon-small-smile.gif" border="0">