It can be to reduce inflammation, not just for the radiation, but for the tumor. Dexamethasone, prednesone etc. When was her last doctor appointment? She should head back and get another scan and have them examine her - just in case. They can reassure you if it is side effects, meds, or something else.
She was never on steroids. Is this something usual for brain radiation?
Has she been back to the doctor? Has she had he thyroid tested at all?
My cousin, dad and uncle all had brain radiation - and i know others that had it done in other areas of the body, and they all were tired - they need to heal.
Sick to the stomach can be the meds, the illness, so many things... not sure about thyroid. Is she still on steroids?
Radiation can have a lot of side effects that happen later - necrosis, etc. so your family member should be monitored for changes.