Only the writer has asked for advice.  Although it didn't seem like DE had any today.  I too have difficult inlaws.  Now that my MIL is 92, she is less demanding and more in need of help from us, so it is easier now than it has ever been.
But I would recommend the wife spread kindness along.  Help with yard work, or cook a pot roast and share the leftovers etc.  Actions speak louder than words