MIL's Behaviour

My partner's Mum is in poor health in her late 70's. A couple of weeks ago she had a go at me for pronouncing the word 'drawing' wrongly. I felt like a five year old after a firm ticking off, and to be honest I was gobsmacked. She prides herself in plain speaking and tells anyone exactly what she thinks, regardless of whether she has been asked or not. I have put this down to just her, but yesterday she was recounting that she had told her grand daughter, who has mental issues and put on a lot of weight recently in no uncertain terms that she was fat and needed to go on a diet, and how gorgeous she used to be and how she has let herself go. I defended the GD and said how hurtful those comments could be and would certainly not encourage her to lose weight. My MIL says ' now I am this age I can say what I like and I love upsetting people!' I said, how would you react if someone spoke to you like that? I am finding her increasingly hard to cope with and it is very hard for her son, my partner, who is on the receiving end of this kind of nonsense far more than I am. Surely bad manners is unacceptable at any age, especially if it is done with full awareness! Sorry to rant - trying to figure out how to deal with it.