Radnurse I am very sorry for you too.
One thing I can say, coming from a family of alcoholics, is that it is not unusual for them to not talk about it. It is "normal"for them to behave in a way that others may not think is"normal." These are the roles they have been in whether they are the drinker or not, everyone has their role to play and they don't know how to stop playing it. I think it is too soon to bring them out of their shell. But when your mother-in-law is less catatonic, or at the next family gathering, which will hopefully not be a funeral, find a nice picture of your sister in law, and get them to talk about a fun memory, or a good time during her life. Talk about the things she was good at. Just kind of slip it in. It won't happen overnight. But from reading Ann Landers and DearAbby every day, people do not want others to act like they shouldn't talk about their loved ones who are lost. As long as she doesn't think you are judging her dtr or her. She, and everyone in the family, probably feels guilty for not being able to stop her, even tho there is nothing another person can do to stop an alcoholic. When my dad died I told my pastor how guilty I felt for not seeing him more or calling him more (I was 16, and parents were divorced.) He said to me, Linda, he didn't pick up the phone to call you either. It is not your fault. I can't tell you how much those simple words helped me to not feel like a horrible person for the rest of my life.
Maybe I am way off track and I certainly don't mean to sound preachy, but you asked, and from my personal experience, this is what I can tell you. I will be keeping you and your family in my prayers.