Trish I am so sorry to learn of your sister's bc. When all is said and done that, and the treatment, is the most important thing. You are obviously a tremendous support for her, and rightly so as she is your sister.
Your father has decided to live his life on his terms and unfortunately there is nothing you can do about it. All the fighting in the world is not going to change his point of view. Sounds harsh, I know, but it is what it is.
I learned many years ago that I cannot change people. I can only control my reaction to them. Sometimes easier said than done but with practice, and time, you will learn to control your reactions.
Sometimes we get caught up in our expectations of people and when they let us down we're devastated. Please do not let this happen to you. Try to focus entirely on your sister as she is the one in need. In time your father may come around, or he may not, only time will tell. You, in the meantime, are taking the high road and everything will eventually work itself out one way or another. Please continue to post and let us know how we can help you. Good Luck & God Bless..........wendyr