I agree with Donald Duck in that I question why the MIL has to go to such degrees (submitting a letter to advice column) to prove her point. I think MIL should spend some time on Therapist's couch in order to learn about focusing on things she can control and exposing the roots of her deep-seated resentment so that while she can in future observe her reaction, she doesn't have to spend time obsessing about it in order to figure out what to DO about it and then go about FIXING it.  Better to put her time and energy into helping at a homeless shelter or something!