Amelia,
You didn't mention if your sister had radiation as well and if so, how long ago. I found that certain products helped at different stages of the healing process.

I currently use Biotene toothpaste, mouthwash and gel (now 8 months post radiation) but early on, I found that the Biotene gel was too thick for the dryness of my mouth (even if I warmed it up some by running it under hot water) and used a product called Salivart. Eventually, my taste buds changed and I could no longer stand the taste of Salivart. Then I tried MouthKote Oral Moisturizer and something generically called Saliva Substitute by Roxane Labs. Each time my taste changed and I couldn't use those products any longer. I ended up settling on Biotene.

My advice would be to experiment and find out which products work for you. When they start to taste funny or stop working, try something else.

Best wishes, Sheldon