As you mentioned, the treatments your mother in law are receiving are very fatiguing. Most anything can be affected by this kind of fatigue including speech. However, any kind of neurological change in your mother should be reported to her neuro-oncologist.
In terms of managing fatigue during and after radiation+chemo, my wife slept a lot, and I believe this was the best thing for her. I encouraged her to sleep anytime she looked like she was tired, and she often didn't need my encouragement. Fatigue and sleep are very normal side effects of this kind of treatment.
I certainly wish you both continued success in treatment and management of the treatments side effects.
Brandon.