Try talking to your sister and letting her know that your daughter will remember her for so many wonderful reasons, the book isn't for that puprose, it is just so the child will understand better. Teen years are difficult enough, and when you add in CF, it is even harder. Try to help your sister understand that CF is just one thing, one part of her. It isn't her whole life. Maybe tell her some things that your daughter loves about her, and tell her that is how she will be remembered. "As the aunt who loved me" or the "aunt who read to me and played with me" who just also happened to have CF. I hope this helped. It is hard to put my thoughts into words. I think she will come to understand the reason you got the book. I would keep it. It seems to help your daughter. Maybe just not get it out in front of your sis for a while.
Good luck.