What is wrong with the younger sister?

She had cystic fibrosis.

Cystic fibrosis; it's genetic and diagnosed in infancy. The average life expectancy is 34 yrs; the lungs fill with thick mucous that has to constantly be suctioned and loosened; the vest you saw shaking her body is designed to loosen secretions to be suctioned away.

Did you not watch the heavy handed scene where the sister says "Do you know what the part of having CYSTIC FIBROSIS is? Having to watch your friends die"

I have a child with cystic fibrosis.

Basically, it is thick mucous throughout the body which causes blockages. The main worry is the lungs, as it can breed bacteria which kill lung tissue, so physio is needed. Think of a pond or a fishtank...if you let it sit still, it goes murky and gets a layer of sludge on top...keep it moving around and it help to keep it clear and fresh....this is the idea of physio, whether its whacking on the back (called percussion) or vests, deep breathing techniques...etc.

It can also affect the pancreas, as the organ may not be able to secret enzymes, which break down the fat in your foods to absorb it. So there is heaps of medication to help digest.

It can also effect kidneys and repoductive system.

Average lifespan at the moment is 37yrs.

Shoot! Bella beat me to the soapbox. As a CF patient of the ripe age of 27, I have been blessed with amazing health. But unlike Grace, I didn't have any CF friends until I was in college (thanks Facebook). When Grace says that line to her father, I teared up a little.