I lost my father to stomach cancer last year. Yes, he had jaundice in the end - and also aspirational pneumonia, which was hard. The doctors told me that he was not in pain, as the tumor was not touching any nerve. So it is possible that your sister also won't have pain.
My father also had problems with bile - constant hiccups because the bile tried to get to his stomach and found an obstruction. He had a J-tube, was very alert. Could hear everything I said. But was weak and on ventilator for the pneumonia. He could hear everything I said. He couldn't speak due to ventilator. When I asked him to blink twice to show he understood what I said, he did.
At this time, be there for her - and show her your love. If she has concerns (like family, young children, other concerns), do reassure her and tell her that you will take care of everything - and she can let go. That she is going to a peaceful place. My father passed away peacefully.
Take care, and God bless.
Priya.