My mom is a nurse. She tries to mind her business, but she says things like, "I don't think you should eliminate any food group." The only thing I can say is that grains aren't a food group for humans. It may be starting to make sense to her. Every time we get into it, I stretch it a little further. Like rehabbing a stiff joint.
I don't know if that will help, but it's how I approach it.
And I also find that people who are close to you are more likely to be open to new ideas if they don't come from you. Because you're so close to them, it's hard for them to absorb your change. If you get their doctor, or someone else to say something similar to what you do, it helps (as long as it doesn't seem as though you orchestrated it). Try to get them to watch Fat Head if they won't read PB.