I am very sorry to hear about your sister. That must be very frightening for you (and your family). Just to share - my cousin (female) sounds exactly like your sister. Never wanted to "get doctored" as she phrased it. Workaholic, bright light and everyone loves her. Smoker as well. She started getting dizzy and finally relented to a doctor's visit. She had Ménière's disease. They were able to treat that, but they also found a brain tumor. It was benign, but because it was wrapped around a critical artery, they had to take it out, otherwise they would have just let it stay there and monitor it.
I am hoping that this is something similar to what your sister has. There is hope! Take care