D.D.
It's amazing to me that some of the craziest people call themselves Christians. Christ said, "you will know my people by their fruit - my people love each other" (paraphrased). In ancient Christianity the virtues are:
1. Humility 2. Liberality
3. Chastity 4. Mildness
5. Temperance 6. Happiness
7. Diligence
The 7 Grevious Sins are:
1. Pride 2. Greed
3. Lust 4. Anger
5. Gluttony 6. Envy
7. Sloth
It sounds as if she has a LOT of problems with pride and anger. Pride showing itself as she knows everything better.
You can actually participate in her "sin" by being silent instead of doing something about it. So don't feel bad about protecting yourself. Even proverbs says not to associate with an angry person.
Slander, gossip, meanness, judmentalism will only hurt the person doing those things. Believe it or not, it's worse for her than it is for you. The new testament speaks against those behaviours. Maybe you can write HER a note, which talks about all of these things that includes scriptures. Holding a mirror to her behavior by showing her scriptures which is what she is doing to you, may reverse the tables on her and bring her to think about what she's doing. In this way could be beneficial (there's s slight chance).
I have known several people like this in the past. The only thing you can do is remove yourself from their presence, and pray for them. Hopefully God will open her eyes to what she is doing. However, don't be an enabler to her behaviour. Like another person said in the responses, "a woman and a man shall leave their household and become one flesh". Anyone who tries to divide that is working against God.
God bless you. I've seen other couples in your circumstance - a dear friend I had in Arkansas. I tried to help, but discovered the hard way, that the Mother in Law would attack everyone - including me. One day I lost my temper in return and told her EVERYTHING I had been feeling about what she had done and what her slander was doing to everyone and I never saw her again! Your MIL obviously has a big problem with unforgiveness somewhere and it's coming out of her as poison. She needs healing and counseling.