Thank goodness for sisters. I was in your sister's shoes when I finished treatments. It took a while to find the right meds and then for them to kick in. In the meanwhile, therapy, meditation, and Yoga helped me. Crying may be disturbing and difficult for you, but it really helped me to clear the air. I actually would listen to music that would help me cry.
Just keep supporting her by being positive. This is a time of baby steps for your sister. With time it does get better and her fear of recurrence will subside.