My husband's doctor gave me a prescription of Ativan that I was to give him under the tongue if he had clusters of seizures or get's very agitated. The slurring of speech and trembling may be a seizure or may be the result of the radiation or a sign of fatigue or or or. Talk to the NO and given your concern and involvement ask your sister's husband if you can go to the doctor's appointments.
My husband's doctor will answer email questions but I don't want him to be overwhelmed by questions.
The biggest think during a seizure is safety. Not hitting the head on hard surfaces, falling down dangerous places, lay the person on his or her side to keep the airway free and talk softly and reasurringly to the person.
Take care,
JMB