If it were me, I would start her on enzymes. Do you know her cf genes by chance? If they are commonly known to cause pancreatic insufficiently like delta 508, I would think enzymes should help. She is well below what is considered acceptable for a cf child (below 3%, right?)
My husband and I were also very thin as children and teenagers. I remember telling my childrens cf dr and GI dr this quite often when my daughter was a baby but they wouldn't listen. After years of struggling with my daughter's weight, I finally got it. I didn't matter what we weighed as kids it mattered that my daughter had a good weight. And yes she could gain weight and be above 50% even if we weren't. My daughter hung out around 20-25% bmi then dropped to -5% bmi when she turned 5. Then she got a feeding tube. I know that sounds scary to all you new moms but it really isn't...it is a huge blessing. She has been around 80% bmi for the past two years thankfully since receiving the night feedings. It is so hard to see them lose pounds when they get sick from virses when they are already too thin. The extra weight they have is important when they get sick.
Why not try the enzymes and see if it helps her gain weight. It is such a struggle to gain weight...why not use everything you can to see if that helps her?
Sharon, mom of Sophia, 7 and Jack, 5 both with cf,
Grant, three months no cf