Good morning Sarah, This sounds like such a toxic, desperate situation. I'm so sorry you are dealing with this. It is so painful to watch our loved ones give up their potential and sacrifice their well being to addiction. Your sister is going to have to reach a spot where SHE wants help. By the sound of it she does, but then goes back...
For a lot of years I thought I could change the outcome for my son. I poured energy, love, time, resources, whatever I could towards him in an effort to "help". I was completely paralyzed by fear. The problem was...he didn't TRULY want my help. He would say he did...but oftentimes I'd spend days lining up this or that only to be told "oh, I'm fine now, we don't need to do that now" or simply "I'm not interested anymore". Which now, looking back, he didn't want solutions. He wanted an immediate fix.
Watch your sister's actions....not her words. Does her behavior indicate she wants change? If so, tell her what your family is willing to do to help her when she is ready. And then hand it over. I've heard it say "never deny an addict their pain"...which, to me, means let them feel the weight of their consequences...but let them know when they are ready for REAL change (not a bandaid) that you will be there for her.
I have set down my end of the rope. It's the hardest thing I've ever done. However, my son knows when he is ready for change and begins showing signs of wanting to climb out of the hole he's in...I will pick up my end of the rope to help him out (standing offer of rehab)...but from there the work has to be his.
I hope this makes sense, keep reading, keep posting, and most of all....know that you are not alone. Many of us can relate to the powerlessness you feel. Big hug to you today.