Sarah,
Don't worry, I had similar experience during my chemo-radiation treatment 5 years ago. I was admitted to hospital for high fever and low white bood cell count. On my first night of the hospital stay, my temperature was high as 104 degrees and I was treated with antibiotics through an IV to lower it. The nurse had to place an ice pack on my forehead to lower my fever. I was only half conscious that night and in the next morning, the fever went down to 100 and I could receive my radiation as usual. This lasted for a few days with high fever at night and low in the morning and according to my oncologist, radiation need not discontinue as long as my temperature was not higher than 100. So I didn't have any interruption to my treatment. It is a wise choice for your sister to be admitted to hospital as she can be closely monitored by her doctors and they should know if the treatment can go on or not. Anyway, they should be giving her the best possible medical help now.

Karen