My W's sister is bipolar, my W suffers from depression and MIL dances on the fringes of bipolar (my assessment, not a doctor's). Part of me has tried to view it from the standpoint that maybe she is sick and in the fog right now. But if she is, she is certainly maintaining enough mental clarity to hire a lawyer and move out. I have always held that some of this is biological - but if she refuses to see it that way there is nothing that I can do until she does.
It's sort of a double whammy - being unhappy with things in a marriage can make you feel hopeless. Depression can do the same. Layer the two things on each other and it is enough to put up a pretty high and thick wall. I am still trying to get through it.
