My mom was diagnosed with breast cancer two weeks ago. Last week she had a lumpectomy. Time will tell whether that will be enough to keep her cancer under control.
My dad died of cancer in fall of 2011, after a four year battle against it. It's hard to say what actually killed him, the cancer, which by then had spread to his brain, liver, lungs and pancreas, or the chemotherapy drugs and radiation that burned and poisoned him to the point that his immune system was no longer functioning and he couldn't digest food.
Early in my dad's cancer I tried talking to him and my mother about the benefits of dietary changes, particularly switching to a low-carb, high fat diet. They thought my suggestion utterly absurd, and had bought into the idea that fat was bad and sugar and starch harmless. Nothing I could say would even open their mind, let alone change it. I offered books, at first they let the books sit on their shelves, eventually they threw them away. Lost my first copy of Nourishing Traditions by giving it to them, along with my copy of Mary Enig's Know Your Fats. So it goes...
Until someone's ready to open their mind, nothing you can do or say will force them to. And if you try to force it, you'll just create animosity. If someone's set on standard medical treatment, stand back and let them use it. Mention that you've heard of alternatives, and if they ask then offer more details, but otherwise don't interfere or start arguments. I also suggest that you start your grieving process so when they pass away, whenever that might be, you can stand strong and offer support to other family members who will be beset with confusion and sorrow.