Oh my dear friend... I'm with SoberLeigh about holding my breath reading this... but glad you shared. I'm so very sorry to hear this.
Parts of your sister's story remind me of a few people I know, an old ex from about 15 years ago, for example, or my father, or some aspects of myself (when I was younger). So I kind of understand her attitude and behavior, I think.
I am sure you know how it's best to interact with her in this situation. What I usually find a good way with similar people who are close to me, in situations when they suffer something that directly cuts into a very old an ingrained resistance is to be there for them as much as I realistically can, and be as open as I can, formulating my worries but without pushing them in any way. It sounds like that's what you have done with her already. I hear you on the adversaries often bringing previously unexpected and unimagined strength to relationships -- it's a sad but very real.
Brain tumors are not something that can be prevented or avoided even with rigorous and regular medical care, unfortunately they are unpredictable chance events. There are also many kinds, not all of them malignant, and many often respond to treatment well. Your sister will know more about all this and the options as she progresses with tests and consultation with the doctors.
I'm here for you, my friend. We all are. Unload as much as you like and whenever here and anywhere else that you find helpful.
Hugs to you... and good thoughts to your sister and family.