First of all, from what you've said in your post, when she is not drinking, your sister is "dry," not "sober;" there is a huge difference.
Secondly, clearly she is not interested in "being there" for herself or for anyone else, so I'm not exactly sure why you feel you have to be there for her????? Personally, I have found that it is frustrating, dangerous and a very poor use of my time to actively care more about people than they care about themselves. I mean, I can love them, and I can pray for them, and I can hope for the best for them, but if they aren't willing to do their part, then I don't rightfully even have a part to do as far as I can tell.
Finally, I do not allow anyone to treat me with disrespect -- and I recognize no caveat to that intention for people just because they happen to be blood-relatives, regardless of how close. I have in the past stopped taking calls from and/or talking to family members who were engaging in poor and/or disrespectful behavior towards me. Now, since I am pretty good at setting boundaries, most people have been able "to hear" me when I say "no contact, but there have been times when I've had to change my number or block certain e-mail addresses, etc.... I will do what I have to to make sure that my boundaries are respected, and I will certainly do so again if the need arises.
I've said it before, but it seems to always need repeating: we teach people how to treat us, and, thus, we have both the right and the responsibility to teach them to treat us right or to get them out of our lives, at least insofar as is necessary to ensure that we are not going to be affected by their bad behavior. So, if someone is disrespecting or abusing us repeatedly, there comes a point pretty early on in the situation when we bear as much responsibility for it as they do, and we have just as much power to put an end to it as they do.
Do what you need to do to take care of you and to make it clear that you deserve and will have respect.
freya