In a very similar situation, it helped me to really listen to my heart. There is a small voice within that tells us why we want to do certain things, and although it's really hard to hear in the chaos of everyday life, getting down to where I could hear it (through counseling, meditation, etc) helped me make "true" choices for myself.
There were times when I received similar apologies and felt an urge to respond in order to try to (benevolently) control and manipulate.
There were others when I realized that I was simply protecting my future self from guilt.
And still others (later on) where I truly felt I loved my sister and wanted to be honest with her. My half of those conversations were something like this: I love you a lot. Thanks for apologizing, because you really hurt my feelings. If you ever do it again it's likely I'll sever contact permanently, for my own peace of mind, so please don't. I think we both know that it was your abuse of alcohol that led you to that place, and I hope, for your sake, you can find a way to help yourself.
And when I hung up the phone, I had no guilt, no fear, no wishful thinking, no sense of having pushed her in one direction or another. I fully knew that she would probably drink again that night. I knew that what I'd said was true: that the next time would be the last time. And that that was okay. I was just speaking my heart's truth, FOR MY OWN SAKE.
Only you can know what your own motivations are. I hope you can find some space in your life to find the right path through this. Sadly, nothing you do can save your sister. All you can do is save yourself.
Sending you hugs - I remember all too well being in your shoes. Twice.
Love,
GL
:ghug3