Buffalo,
How long will you continue to be the scapegoat? How long will you question your need for your own sanity? How long will you continue to try to live in this very sick dynamic?
IMO, there is no "right" and "wrong" here; there is only blame. She blames you, He blames you, and You blame you. Allowing yourself to continue to think in terms of right and wrong will just keep you believing and participating in the blame game. Try instead to focus on stopping yourself from thinking in terms of right and wrong, and start thinking in terms of 1. What do I NEED? and 2. Is this particular thing going on helping me or hurting me? Allow more of what helps and eliminate what hurts.
I wish I could let you feel the ABSENCE of all this for just one day. I know you must be exhausted, not just from the lack of sleep, but from the thinking about all this.
My BF's sibling did to me what you describe MIL has done. Started yelling at me and accusing me of things related to BF's family, which really had nothing to do with me. Attacked my character and my values. I simply walked away from her. I have reached a point in my life and my Recovery where I am finally able to not internalize it, not react, not let it make me mad, not let it affect how I feel about myself. Because I KNOW myself so well now. I am too old for this type of chaos in my life. I have had to let go of a lot of emotional attachments to a lot of different people and of varying ages (children through adults) in order to gain my sanity, my serenity, and my peace.
(((hugs))) Let Go hon. Remember: Sick people will make you sick.