I am so sorry. I don't think you can do much more than you already have. Just keep listening and being there for her. Maybe point her in the direction of SANDS or the bereavement board on here. It might be easier to talk to someone who has also lost a child. Tamba also offer support for bereaved parents of multiples. You sound like a loving, caring sister.
My sister recently had a miscarriage of twins, it was all very complicated and they were aware that due to treatment she had to have there was a high chance it would happen, not that it makes it any easier but anyway unfortunately it happened. Obviously they were devastated but she is not a big one for showing emotions and although we talked about it afterwards, I just mostly listened, re-assured and told her I was here if she needs me, Im still convinced she is still bottling a lot up. Any advice?