Hi, I'd like to help you with your question.
It is always difficult to help someone who is struggling to fight a battle like Anorexia. On one hand, you feel you want to do something but on the other hand there is not much you can do. The person themselves must do most of the work. But there are some ways you can try to be there for your sister:
1. Don't bring up your sister's size or how she looks- this is the main focus of an anorexic so bringing it up just reminds her that she is successful in starving herself.
2. If you live near other family members who are involved with your sister's situation, consider family therapy. Your sister may be able to attend when she visits, but the main issue should be helping you and other other family members cope with the stress and worry.
3. Educate yourself as much as you can about anorexia. The more you know, the more helpful you can be to your sister.
Here are some resources to help you:
Anorexia Nervosa: A Survival Guide for Families, Friends and Sufferers by Janet Treasure
Anorexia and Bulimia in the Family: One Parent's Practical Guide to Recovery by Grainne Smith
The Anorexia Workbook: How to Accept Yourself, Heal Your Suffering, and Reclaim Your Life (New Harbinger Self-Help Workbook) by Michelle Heffner, Georg H. Eifert and Steven C. Hayes - for you or your sister
You can find these books on Amazon.com or your local library may have them for you.
I hope this has helped,