The best thing you can do is to be Sympathetic. Simply put, sympathy is "feelings of pity and sorrow for someone else's misfortune." Wikihow has a pretty good walkthrough of ways to show sympathy for someone. Since you are unable to see them in person, I'll only outline the verbal (could be used in text as well) options.
Be direct and acknowledge their pain
I'm sorry to hear about your sister. I can't imagine how hard this is for you."
Whether you have had a similar experience or not, it is important not to say something along the lines of "I know how you feel". When I was younger I had a friend whose father died. My mother explained to me that I should not tell him I know how he feels, because that can be very offensive to someone who is experiencing grief. As I got older and experienced loss myself, I found that the last thing I wanted to hear someone say was "I know how you feel". Everyone experiences grief differently, so it is best to simply acknowledge that they have pain and you are sorry that they have the pain.
Make yourself available
If you need to talk about anything, I'm always willing to listen.
Don't push them for information. As I mentioned earlier, everyone grieves differently. Offer to listen if they need it, but leave the decision of whether or not they do up to them. If they decide to talk, just listen. People who are grieving rarely want someone to offer them advice on how to cope, but they may want to talk through the pain they are feeling.