I hope your sister's first day of treatment went well yesterday.
I was like Kelly and did not have terrible side effects like some people do. I did not have a PEG - I was determined not to. So I drank alot of Ensure, Boost, Special K Protein, Carnation Instant Breakfast throughout the day and my husband made lots of shakes for me to help keep my weight up. They also gave me swallowing exercises and facial exercises to do. I started them before treatment and continued throughout treatment and I think they really helped with my swallowing and tightness of my neck and mouth. I used AquaPhor (over the counter) on my neck to soothe the burning and help with healing. Also soda/salt mouth rinses several times per day to help with mouth sores. Whatever the doctors told me to do, I did. I did lose my taste which was not fun but started regaining it about 8 weeks after the end of my treatment. I can taste most foods now but not as intensly as before. My mouth is always dry and I keep water with me always. I use fluoride trays every night and I have regular appointments with my dentist to keep my teeth in the best shape possible.
I know at first I was hit with everything that possibly could go wrong but it doesn't necessarily happen that way. It's not a fun process to go through and life is different but she will come through treatment and things will eventually get better. Hang in there with her. It really helped me to have my husband with me and guiding me and telling me what I needed to do next because at some point I was just tired and didn't want to think about anything. Keep us posted.