I wish your sister all the best flump and hope that she gets that call soon!
My husband has had 2 kidney transplants over the years and I believe with his last (9 years ago) if he hadn't have had it when he did, he was getting so poorly that he wouldn't be here now.
I find it a very emotional topic when it arises when people give their reasons for not wanting to be an organ donor. Your right, if they or a loved one needed it and they would take it, then if their prepared to do this then they should be on the register.
Don't get me wrong, the realisation that someone has died when you get that phonecall is actually quite horrible. I know it is not something I had thought about until it happened and all I could do was cry for the family as I felt guilty we were getting what we wanted/needed and they had lost a loved one.
I am eternally greatful to them, every year just before the 4th of July (when hubby had his transplant) I think about that family and know they are thinking about the anniversary of their loved ones passing.