I just wanted to say that my thoughts and prayers are with your sister, you and your family.
I was in the same spot a little over 6 months ago with my 53 year old mother. It was the hardest thing I ever had to do and I wasn't even the one actually going through it. My mom's first sign the end was very close was the vomitting of bile. It was so traumatic for her. I know a lot of people write about food and drink becoming unwanted near the end but, in my mother's case she was nourished with a feeding tube. The cancer ultimately stopped that from working and she chose not to continue with iv methods. I was given books to read about other signs the end was near such as the color of the urine darkening, the skin color (it might be called mottling?) and the breathing would change. Sometimes her breath would stop for what seemed like a long time but, then would continue irregularly. I saw these changes all happen in the last 10 hours of her life.
I truly hope the doctors and nurses find ways to make your sister as comfortable as possible. Again my prayers are with your sister and family.