Some really great answers here so far. You know, it's a hard answer to give you, luvmysis, because it depends on your sister. Having this diagnosis in and of itself is very overwhelming, so I'd offer support when you see a need for it. Keep doing the things you have always done as a sister, and let it be known that you can do more when she needs it.
I'd also say, if she likes Facebook and the like, show her this site. I think it's been really helpful for me in my new journey with cancer.