There's some information on my profile page you may want to read. Just click on the blue GaGuy link to the left of this post. Also, google Suboxone and read up on it. It may be a choice for your sister.
Your sister is blessed to have you and her husband. THAT is what will help her more than about anything.
She may need to get into an addiction treatment program. This is a hard step as I just took it today but it is mental just as much as it is a physical need.
Just be supportive for her, love her and tell her you will be there no matter what and believe me, it will go a long way in getting her through this. My mom, my aunt, my grandma, and my husband (now...at first he just ignored me as he didn't know what to do...and he was scared and having to take care of our two little ones alone so also overwhelmed) have been here to help me and help with the kids and without them this would have been even harder than it is.
Your sister, along with you and her husband are in my thoughts and prayers.