When Alex was first diagnosed, I jumped on the phone to the only other person I knew who had suffered through oral cancer and chemoradiation (didn't know about this website at the time). He was in a different country and had different names for things but alot of what he told me was relevant.

The lesson that stands out most strongly was the assurance that we would have days or even weeks when Alex would either threaten to or actually refuse treatment. His advice was to gently probe for the reasons which may not be obvious to Alex himself. So questioning would be non confrontational or accusatory, a little bit airy fairy and VERY gentle (this is definitely one time when nobody expects a Spanish Inquisition). Another respected person to have a chat other than the caregiver may also be a good idea as the new perspective may uncover things we haven't considered. For my friend and for Alex, it turned out to be pain (bloody men). As soon as this was communicated to the doctors (after overriding Alex's stoic but unhelpful "not too bad" response)the pain meds were changed and or increased and treatment continued until the next barrier was encountered. Depression was the next thing caused by one of the pain meds (morphine) so that was changed.

Don't know if this is where your sister is at but at least it gives you something else to try.
Karen