CindySue, I understand your frustration with that. Here you have the knowledge to help her achieve much better control and minimize the potential for long-term complications, but she's choosing to listen to "an expert" that you believe (and I agree with you) is dead wrong. She's your sister and you love her and you don't want to see her heading down the path that you know she's on, but she's still free to choose that path whether you like it or not.
Perhaps if she won't listen to you because you're "just a nurse" , perhaps she will listen to a few doctors on the matter...like Drs. Bernstein and Schwarzbein. Any chance you can get her to read a few books written by doctors who are specialists in treating diabetics?