These are very toxic and manipulative individuals who divide to conquer. The advice I have had from professionals is to walk away. You cannot do a thing to change them and if you attempt to interfere in the family dynamic they engineer, you play into their game.In my family, even the grandchildren eventually had to just walk away.
Your wife and children are probably the ones who need the healing. It is easier for the in-laws.
“Divide and conquer” that sounds like my husbands mother. She has been dead for 30 years and did not really impact our children that much. Thank God!
I did as you say, “walk away”. She and I had some good times together, she could be charming and generous. when she wasn’t dividing and conquering. She had been dead a year before I could grieve a little bit for the good times. I remember at the time of her death I felt nothing.