Welcome to SR Jacki. It is so difficult to watch a person self destruct. You sound like a very caring person. There are a lot of people who have dealth with advanced alcoholism and can give you some insight. There are several good books on alcoholism....one that comes to mind is "Under the Influence" by Milam & somebody else (can't think of the other author off the top of my head. It explains the myths and realities of alcoholism.
I hope that your SIL seeks help before it's too late.
gentle hugs