I'm so sorry that your sister is having a bad time...
Magic mouthwash - does help...My sores turned into thrush 3 times...Sores in my nose, mouth and throat...so if it gets bad...she may need a fungicide as well..
Also...the radiation oncologists use a different mouthwash...that is considerably stronger...Kinda like going to the dentist and getting novacaine....So if the magic mouthwash doesn't help enough...she might want to ask her oncologist for the stuff that the radiation folks use...
Anyway...
Thanks for being such a wonderful caring sister...