I want to preface this by saying that even though my future mother-in-law and I aren't very close, we still get along mostly. But she never dresses up. For anything. The most I've ever seen her dressed up, was a shirt, capris, and house slippers. It was quite frumpy, and bordered on embarrassing. But I tried my best to ignore it, and focus on having fun. That was what she wore on a VERY expensive Mother's Day dinner cruise that my fiance took us on this year. He invited my mother too, but she couldn't go because of illness.
Our wedding is pretty formal, and we know that she will not follow the dress code. So instead of putting pressure on her to go shopping, which she hates, and will complain about, we figured that we could pick some outfits for her, and let her choose what she wants to wear from them. My fiance's stepfather is of the same opinion. He doesn't know how to
tell her that the way she dresses, bothers him too. We've all tried
talking to her about it, but she goes off on us, and says things like,
well fine, don't invite me to your damn [insert event/occasion here], or
things of that sort. It's just a very delicate situation all around, and I want to keep the peace.