My heart goes out to all who struggle with this issue. My daughters were diagnosed at 4 and 1, and we decided not to have any more children. I know that if we had found out about Tess's CF before Fiona was born, we never would have had another child. Know that we do know, there will not be a third child, and we have taken steps to make sure that is so.
Yet when I imagine the possibility that Tess was diagnosed before we had Fiona, my heart breaks. Not only is my younger daughter a delight - a truly remarkable, irrepressible spirit - but she is also her older sister's best friend and partner in all things. Because they both have CF, it is "normal" for them. They understand (well the older girl does) that others don't have to do treatments, but they know one other person whose life is just like theirs. While I never would have wished for two children with CF, there is a strange comfort in the fact that they will not face CF alone.
I am not trying to suggest it is 'easy' or 'good' to have two children with CF, but for my family, there are strange and surprising things that came out of having a second child, and our family is richer for that. Yet I am grateful that I never had to decide, and I am grateful that I did have a second child, even though she also has CF.
I am not sure any of this really helps you answer the question you need to answer. You will find your own answer, and it will be the right one for you, because, in part, you will choose to make it so. In the end, that is all we can do, I think.