First, welcome to this site. I am sorry that you and your sister are faced with this disease.
I would like to first suggest that you post on the "Newbie" section of this site as you will get more responses typically from there.
I would say that the best thing you can do for your sister is be supportive. She may not want to talk about it right now. We are all so very very different. Ask her if there is anything she needs. Let her cry if she needs to. Be there for her.
Please keep posting. And tell her about this site. It has saved me! There are so many wonderful women on here. They are all so supportive and informative. This site has been a life-line for me.