Others here have given you wonderful advice (particularly fondreflections). It's a tough spot to be in, but I've been there myself.
When my grandpa was dying of lung cancer, he got a particularly contagious bug (can't remember the name) in addition to MRSA. The potential risk to my health was, unfortunately, too much. For the last 2 years of my grandpa's life, I didn't get to spend any time with him beyond that of our phone calls. One Christmas in particular I was home alone, wisely "banished" from the family Christmas party, and was feeling very resentful. I was dating my (now) husband at the time, and my mom told me that missing a few parties with grandpa could potentially be a trade-off for YEARS of good health and quality of life with Brad.
This is going to be a very, very difficult time for you, especially if you and your grandmother are close. Personally, I wouldn't risk it. You're right to be nervous, and you'll have to deal with this as a sort of loss. It's tough. I know.
Prevention is key. As hard as it's going to be, please consider putting your own health on the highest peg, and limit your physical contact (direct or otherwise) as much as possible. Talk to your CF docs, and see what they recommend as far as cross-infection protocol goes.
I'm so sorry to hear you're in this dilemma. I wish you the best, and for peace of mind for all involved.