It is sometimes hard for people with cancer to be constantly reminded of their disease . . .we do want to talk about other things, too.
You are doing the right thing by giving your sister some space, but continuing to write so she know that you are there if she needs you to help. Cancer patients need some lightness and joking as well as concern; it is just too hard to think about the cancer all the time.
It must very hard to be far away for both you and her at this time.