I agree with Deanne. There's nothing like clearing out your living space to help you clear out your mind. One's home and one's mind can so easily become cluttered with a lot of old rubbish.
Your preacher brother-in-law is wrong. His experience with being a living donor gives him no special insight into when it is a good time to get a transplant. I have never read any medical literature that explores the correlation between letting things get "really bad" and the risk of rejection. What does he mean by "really bad", anyway?
Your sister has to deal with her demons on her own. She feels guilty, so she is hoping that speaking to your other sister will somehow absolve her. She cannot speak to you without risking further humiliation and guilt; she lacks the personal courage to speak to you directly. It is good that you are moving on because what else can you do that would not cause yourself more harm?
Do you think she would have passed all of the tests if she had chosen to proceed? I know I'm asking you to merely speculate, but what does your gut tell you?