Nicole,
I agree with Marjolein that 18% is not a lot of reserve and that your sister should really look into getting evaluated for the transplants. I started my evaluations in June 2005, found out that I was pregnant in August 2005 and stopped the evaluations, and then started them back up in March 2006 after the baby was born. From March of 2006 until my double lung and liver transplant in January 2007, I lost 22% of my lung function. I was at 37% and dropped down to 15 percent. I dropped more than what your sister has left and that would scare me.
There are definitely risks after transplant but I knew that if I didn't get one that my risks of not getting one would be a lot greater. If I did not have the transplants, I would not be here for my son today. The better shape that your sister is in when she needs the transplant, the more likely she is to do better after. Even though I was at 15% lung function, I was still moving around and taking showers and also trying to take care of my son. That is likely the reason that I did well after transplant. I was out of the hospital 12 days after my double lung and liver transplant and back home 2 1/2 weeks later. It's been almost two years and I would not change any of my decisions at all. I would also do it all again if I ever had to.
I wish your sister the best of luck and if you want to talk some more, send me a PM and I will give you my e-mail address.