Thank you, Atalose.
I let her know that I couldn't keep supporting her financially, or with keeping her furniture when she goes into rehab. I didn't hear anything back from her about it, but today I've heard from our Mum that she (sister) is saying she is rethinking going to rehab at all.
I thought this was probably going to happen, and on a logical level, I know nothing has changed. If she was only going to rehab because of my financial support (she still has well enough government support to go to the rehab), then she wasn't going for herself anyway. Now the only difference is I know that she doesn't want it for herself... And I think I already knew this.
But god, it doesn't stop it from hurting. I'm really tired. I feel for everyone here who has watched their loved ones suffer like this, and be so helpless to do anything about it.