Annette,
The symptoms you have described are the after-effects of (primarily) radiation therapy. Though most radiation oncologists proclaim that these symptoms shall subside within 4 weeks but if you read the testimonials of members in OCF, you will find that patients suffer from these after-effects till 2-3 months.

My mother has completed her RT last week and she has severe pain in her neck and mouth. She has been prescribed to use a gum-paint (x3) and Decdak 4 + Glycerine (x4) for oral administration. Besides, the betadine gargles (x5) are a must to prevent any further infections. She also uses a medicated toothpaste which does not irritate the sores in her mouth. If you need the specifications of these medicines, please let me know.. I am a bit hesitant to write them down here because these could be very different from what you get in US.

Besides, all of these symptoms are temporary and the specialists repeatedly insist that the patient will return to a normal life. It also depends on the patients on how soon these after-effects fade away but please ask your sister to continue her nutrition properly via NG/Peg tube till she is confident to eat orally. I say this because my mother was quite adamant on getting her NG tube removed soon after RT was completed but now she realizes that it would not be possible for her to eat for another month (or a two).