L.K.
I have a BIL and SIL that are just like that. It is so hard to come up with neutral conversation . they are so negative and are socially inept. It is painful to visit with them most times. They like to start fights . I have just learned to live with it and try to have limited conversation with them. I complement their kids alot :) if you have a choice to have contact with her I would do so with others around . You will never change someone like this so don't try to figure her out . live your life and don't feel guilty. we create our own destiny. She chooses to be unhappy. You choose to have positive people in your life . Good luck:)