Oh my Sadsister, what a terrible thing for your Sister to be going through and such a worry for you and her loving family.
I think that a lesion in the right place in the brain could very well cause this to happen.

There must be meds to help. Talk to her Neuro/Psych and impress upon him your concern and worry for the well being of your Sister. Something similer happened to another member here. Haven't seen her for awhile, but the last time she was here, she sounded back to normal.

I'll be praying for her quick recovery from this worrisome symptom.