What can I do to support my mother in law who looks like she is turning into an alcoholic?

Talk to spouse. Your spouse may be more influential with his/her mother than you are. I would enlist the help of your spouse as well as other family members and even close friends. Next, try and make a doctor's appt to have a certain blood test for the liver enzyme: GGT (also known as ggtp). If it is elevated, that is a sign that alcohol is doing damage to the liver. Then maybe the doctor will have some leverage.
Mutual support helps. Consider attending a meeting of al-anon for the families and friends of alcoholics.