Hello Lynd, so sorry to hear about your sister.  I also have stage 4 cancer, so I feel qualified to answer your question to a certain degree.  It is quite usual for large quantities of morphine to be given to a cancer patient when they are nearing the end of their life.  It is difficult to say whether or not your sister would be conscious if she wasn't on the morphine, but it is certainly possible that without the morphine she would be suffering pain.   I have noticed that when I have large doses of morphine, I tend to fall in to a deep sleep that lasts for hours.  In short, it is POSSIBLE that the morphine is keeping your sister in a state of unconsciousness,  but at this stage, the job of her medical team is to make her as comfortable as possible.  

Thank you.  She has managed her pain really well until very recently. She was admitted to hospital for nausea and was meant to come home last Saturday, that day she was sick so they kept her in. Since then it’s been a rapid daily decline ; she’s now unconscious. Thank you for responding and I wish you well on your journey with it