Hello, and welcome.
I can really sympathize with your mother-in-law...
Mouth sores are one of the worst side effects of oral cancer treatments.
Has her Doctor suggested temporary intravenous (PEG) tube feeding?
P.E.G. is short for Percutaneous Endoscopic Gastrostomy..
I too suffered from pain due to chronic mouth sores and ulcers after treatments...and rinses didn't seem to help
...and l tried them all..!!!
Intravenous feeding was often suggested...but stubborn me kept refusing...
However, after a couple of months of being constantly hungry & loseing weight..l reluctantly had a tube inserted and used it for about a year..
When my mouth healed enough to where l was able to force down smoothies & shakes..l ditched the tube.(hated it).
However, it did help me maintain a decent weight, and energy level during that year..which is imperative..

Send my best wishes to your MIL ...and tell her to keep persevering...

Good Luck. !!