I know that this must be very scary for you and your family. You see sometimes when a person has an eating disorder, they are so numb to the world around them that they do not realise how much the people who love them are affected by what they do.
I have one younger sister and I have also ended up in hopsitpal because of overdosing and she was so so annoyed, she really hated me for being so selfish but its not that I was being selfish, I really believed that because I was hurting my family so much, it would be better if I left and died because I thought that would end my pain as well as theirs. Thats not the case but sometimes I still want to die and its very difficult to think of other people when you feel so alone. It really feels like hell, we live in hell everyday, every minute and sometimes it does get too much for us to handle.
Families get affected alot because the person who is suffering is so numb to what they are doing and how they are feeling but its the family who are not numb and they see whats going on, in a way it hurts them more and your sister has the luxury of being so numb but that won't last forever.
I understand where your sister is coming, I understand the hell and hopelessness she feels but she has to realise that she is not on her own and people do love her.
What you can do is let her know that she has really hurt you because she needs to know she does affect other people but you could also tell her that you care so much for her and its killing you to see whats she's going through and although you can't fully understand what is going on with her, you too are affected by it and are hurting too. Tell her that you care for her and that you want to have a big sister again and that you are scared you might lose her. Tell her how you feel, tell her the truth she needs to hear it, if she knows it or not. Your sister needs to be brought back to reality and if it takes you to tell her how you feel she will be upset but appricate your honesty.
Now for you, it is so so important that you take care of yourself, put yourself first, do things for you, you will always be there for your sister but in life we help people by example and if you can look after yourself, if you can continue on doing the things you like, going out with friends, reading, what ever it is, don't forget about yourself and you will help your sister realise that in life, always put your own well being first.
I hope this helps you, good luck and write anytime.
Take care