I am so sorry for you friend, but so happy that she has someone like you in her life. Bless you for that.
I think the best thing you can do is listen to her. Encourage her to talk about her sister - memories and fun things. Focusing on the suicide is good for the times when she needs to do that, but at others', she just needs to talk about her sister in the same way she would if it was a car accident.
She also needs to know it is OKAY to talk about it and that it is OKAY to be sad for a long, long time. People will start assuming that she's moving past it or they will completely ignore the word "suicide." People get very weird about this stuff, mostly because they don't know what to say and are trying to be nice. But it makes the person who is dealing with it absolutely nuts. A SUICIDE occurred and they need to talk about it.
Another good thing is to pick a few books about suicide - like survivor stories, books that explain the process of healing and so forth. She doesnt need books that are "technical" about suicide, but more of the emotional response and how to get through it. I can suggest some if you are interested.
She should also go to a survivors suicide support group. These are so good for people as they try to work through the intense grief of a suicide. They meet people on the same journey and can share in ways that others' wouldn't understand - just like on here.
And she needs your friendship. She needs to know that she can call you at any time - day or night - just to say "I'm so sad" or just to cry. That's all she will want - just to release - and knowing that you are available (or others) makes all the difference. Those late night/early morning hours are horrible when you are all alone and nobody hears your cries.
Family members are at high risk of suicide in the month following a loved ones suicide. They are weak, vulnerable, confused, and in such despair that they, too, decide it would be the easiest route.
Ok - that's it for now. Hope some of this helps. let me know if you need more. This is my "area" so to speak.