wow, 23 weeks. My boys were born at 25w4d and just came home at the beginning of this month. Your sister has a very long and difficult road ahead. Being so far away, the best thing you could do is make sure she knows that she can call you anytime. All of the odds are against the baby. There are going to be a lot of ups and downs and one of the best things was having a few close friends and family that I could talk to anytime. I'm not sure how far she is from the hospital but if you can send money for transportation to and from the hospital or for food while she visiting the baby, that would be super helpful.
Also, there's a good sticky thread about what we wish our friends and family knew about the nicu.
Congrats to your sister! Remember to celebrate, her baby may have been born really early but she gave birth and it will probably be nice for your sister to feel celebrated. Send a congratulations card or a nice preemie outfit (even though it won't fit for a long time) or a nice micro preemie shirt from here http://www.preemie.com/Preemie%20Yums_bymfg_10-1-1.html
I bought 2 micro preemie shirts shortly after my boys were born, they were huge on them for a month or so but the nurses put the boys in their clothes and it made everything more "normal: for me.