Hello @mh2023 and welcome. My understanding is the Chemo sickness is only a few hours to a few days where as the radiation sickness can be steady with increases roughly six hours after each treatment. There are anti-nausea meds that the oncologist can prescribe and quite possibly several might need to be tried before you hit on one that works well. Then the trick is to eat what you can, be it scrambled eggs and toast or custard or oatmeal, whatever works and as much as can be tolerated. Many of us have had to have a feeding tube or PEG tube installed to get adequate nutrition. I for one was on some heavy meds for the last three weeks of radiation and still lost twenty pounds (ten kilos).
Others may have some good suggestions. But all in all make sure the radiation oncologist is informed.
At what point is your family member in treatment?