As has been mentioned, this is a very HOT topic.
Up until last year, I would have been open to the idea of taking the risk and having another child but my husband never was. Over the last year, I've rethought it and I definitely wouldn't do it. This really is your (and your husbands) choice but I encourage you to think out every possible scenario.
Yes, the odds are 1 in 4 that your next baby will have CF but to make an informed decision you need to assume the baby will have CF. Assuming the baby will not should not play into your decision because having a baby with CF is really what you are questioning.
Please consider the following (if anyone can think of anything else, please add)...
1 - The new baby may have a more severe case of CF. Are you prepared to explain to your older CFer why this 2nd CFer is not thriving and could potentially die before your 1st CFer?
2 - Are you prepared financially? If your financial/insurance situation is ok/good now, there of course are no guarantees for the future, can you handle an increase in the cost of care for two CFers?
3 - Do you have the time to put into another CFer? As you know, CF is time consuming from a parents standpoint when the children are young. Can you still give the appropriate amount of attention to your current children while caring for another CFer (again, you must consider the possiblity that the new CFer may need more care than your current CFer)?
4 - Have you discussed this with your existing children? What are their thoughts about another possible CF baby? 11 year olds can be quite insightful and honest. I'd definitely ask your CFer how he feels.
5 - Cross contamination is definitely a consideration. Discuss with your family what your intentions are for cleanliness, disinfection, separation, etc.
6 - Can your family handle the emotional side of having another CFer? Open and frank discussion with everyone involved may help you come to a decision.
7- Are you prepared to explain to CFer #2 why you decided to gamble with his/her life? When he/she is older, he/she may not be thrilled with you for knowingly "giving" him/her CF.
Have you considered IVF w/ PGD? If you really feel like you want another child you really should look into this.
Please don't disregard the feelings of your children. Yes, the decision is yours to make but at 12 and 11, they may have some very strong feelings about this. If you choose not to solicit their opinion or choose to disregard it, they may hold it against you.
Best of luck.