I was given chemo once a week and radiation 5 days, but at the same time, so I'm not sure this is the same as your sister. But, I suffered nausea after radiation treatments. Maybe it was the after affects of the chemo kicking in (they give you anti-nausea drugs during treatment) a couple days later. I'm not sure. Towards the last 2 weeks of radiation, I was taking anti-nausea pills a half hour before treatment. If not, on the drive home from treatment I would get real sick. Hope this helps.