I really don't have any suggestions except above all else "force" her to go see a doctor. Whatever you have to do, get her to go. Pretend you're taking her somewhere else, bribe her, tell her that you guys will never speak to her again ... whatever it takes ...
Last year after not being able to get my mom to answer the phone, or the door, we went over to her house and broke out a window to get in (she had changed the locks). I found her in her bed dead. She was 62. Like your mom she had become mean, intollerable, and very defensive. It got to the point that if you said hello, she'd ask why you were speaking to her. So needless to say it was VERY hard communicating with her. Also like your mom, she had not been to see a doctor in many, many years. Anytime I mentioned it she bit my head off. So eventually I stopped asking, but I felt guilty because being an only child I felt as though it was my responsibility. I guess I should have listened to that guilt, but we get so caught up in our own lives that just getting through the day is a challenge. And not wanting to meet with any more resistance, or deal with any more stress, you lose energy to "fight the fight" - if that makes any sense. So as to not make this into a novel (smile), my advice to you is to get her some help by whatever means necessary. I really DO understand just how difficult that may be, but if anything were to happen to her; wouldn't you want to be able to say that at the very least you did everything you could?
Please let me know how it turns out.