This is just my opinion..... I would not keep it a secret. There are lots and lots of people that will help, cover for her, help her emotionally, financially, etc. Three times I have been through this, and each time I didn't try to keep it a secret, but I didn't go around telling everyone either. Just through those who knew about it, the word got around, I got calls of encouragment and offers of help from people who I thought would never even give it a second thought as to whether or not I lived or died. Now I am clean for the third time and I look for people who have been diagnosed and try to help.
I wish you and your sister nothing but the best. Think about this and talk to her about the pros and cons of letting people know. You will be surprised what happens.