It's hard to help someone that doesn't want to help themselves, or doesn't see themselves as having a problem. I have a sister like that, who by the way called me some inappropriate names just this morning! She refuses to listen, thinks she's always right and has absolutely no morals when she verbally slams people (me being her latest scapegoat). I am at a point that there is nothing left for me to do but; pray for her.
People need to think happy to be happy! I'm afraid if you try to get involved you may be on her "bad list". If your a believer than try praying. I know by trying to converse with my sister when she's in one of her "moods" that alls I'm going to get out of it is a stress headace and shaky hands.
How is your relationship with your mother-in-law? Have you known her for a long time?