Hi LaRue-
Welcome to the site. I know you aren't here for pleasure, but welcome just the same. I hope you find this place as helpful as I have. I have found much support and knowledge here.
I agree with Liza. When my son was dx, I felt like crying all of the time. I still feel that way at times. It just gets to you sometimes. I don't like for people to see me cry. It makes me feel weak, etc. That may be what your daughter-in-law is feeling. Also, she may not want people to cry or be sad in front of the kids. Maybe she is afraid that will scare them.
Everyone deals with things in a different way. Allow your DIL to handle it her way. I know it's hard to sit back and feel helpless, but CF really does make you feel helpless sometimes. There's just no way around it. They say knowledge is power, and for me, that is very true. The more I know about CF, the more I feel I can do to stay on top of it, slow it down, etc.
I also agree that the day will come when your son & DIL need some time to themselves. Just be there for them when they need you. Learning how to do the treatments and give the meds, etc. will be very helpful to them one day.
If you need to vent or ask more questions, just do it. You will always find someone here who understands what you are feeling, or has been through what you are going through.
Stacey