Ugh, that is really sad. Its sad that your mother-in-law won't stand up for herself, and I hope that someday she decides to. Myself, I have a really hard time seeing anyone behave like that (despite what I have allowed my own self to be put through), and I would probably tell your father-in-law how I felt about his rudeness, and that maybe if he wasnt such an arrogant jerk his wife wouldnt have so many gray hairs. That may not be the best way to go about the situation though
just my opinion....I think anyone treating someone else like that should be told its not ok, whether or not they are an alcoholic. There is such a thing as manners.