Jenny,
My first admit was very similar. I was about your sister's age. My PFTs were stable in the upper 60's at the time and I just pooped out. I don't know what my FEV1 went down to, but I was very SOB and my pulse ox was in the 80's upon admit. I had no idea how I got so sick so fast. College will sometimes do that to a CFer though. (I was a 4th year at the time)
My point is, I recovered. It was so "easy" to bounce back then. It's not so easy now, 10 years later, but I'm still kickin'! I hope your sister bounces back. It is possible. Hang in there.