Ellen,
There are three great qualities that you wrote about her in your post. You said she was kind to you, she welcomed you with open arms, and she championed your parenting choices.
In my book, that is not too shabby of a mother in law. She is the one who made the choice to alienate her children and grandchildren, and that had nothing to do with you. You and your children loved her as much as the disease would allow.
You wrote a wonderful post about her and I hope you can find some peace. There was nothing you could have done to save her. Alcoholism is a horrible disease.
Sue