I am sorry to hear about your sister. Truth be told, in cancer you simply can't know. Each case is different. Some start with the early stages but it comes back violently. Some go on to live a long life even after begin diagnosed with metastatic cancer. No one knows how things will develop. We have some statistic but that is all there is. 

You need to find the positive attitude inside of you and take it one day at a time as we simply do not know. Living with ambiguity can be hard but it is something you can learn to live with. Take it one day at a time. Step by step. There is no point in  looking too far i the future because we can not see it.

I lost my sister to breast cancer. Things didn't go well at all but it could be a completely different story. We don't have any influence really on how things turn out. So no point in trying to "control" what is t in our control. Now my husband is in treatment  for his cancer. I have really had my share of cancer in my life. ‍♀️

I wish you all the best. May she respond to treatment beautifully and may it become a positive cancer story. It is possible for sure.