My Husbands Mother In Law Is 86 Years Old, I

My husbands mother in law is 86 years old, I visit her and her husband weekly to check on them and try to assist them with whatever they may need. His health is failing, he has much difficulty walking and becomes easily short of breath. Mom has been doing okay but she is beginning to become weaker in her extremities and is shaking much worse. The main problem is her constant, loud continuous repetition of occurrences over the past year, concerning "dads" health and care. She is also easily agitated, angered, difficult to calm down and determined she is the only right person in all things. She feels no one believes her, listens to her and that all doctors are out to cause harm and only give out pills. She and Dad also yell at each other frequently, she is controlling and tells him he is wrong and interrupts him before he can finish a sentence. She has been very stressed since Dads health has gotten worse, also 3 siblings have recently passed away. She is able to keep up with ADL, keeps the condo clean and has excellent retention of materials she reads. Trying to care for her is difficult, she is strong willed and very set in her ways. I am having difficulty trying to get her to make an appointment with a physician. I am not sure how to help her at this point.