Do your folks know that they are enabling your sister and that they are not doing her any favors? With what's being provided for her, she has no reason to stop drinking. My mom had a wet brain also. At that point, I don't think a person CAN make the choice to put it down. Of course, there is always the Baker or Meyer's Act to force her into treatment.
Just please do not let what she does eat at you. Do not let her "live rent-free in your head." You are the one who is hurting when you do that--it doesn't bother her at all. It's important that you not dwell on what she is doing--or you could find yourself picking up a drink. I really think it is the disease itself that has you so angry...what it is doing to your sister, yourself, and your family. For your own sanity, write a gratitude list of all the things you have in your life that are positive and that you are thankful for. You cannot change your sister, but you can change your focus.
Many hugs,
Connie