I will reiterate some of the advice you've already gotten. I'm relatively new at confronting this issue myself, so I profess no expertise. However, as the husband of an alcoholic wife, I know that having friends and family members in whom to confide is very therapeutic, at least for me. As I'm living with a person not grounded in reality, I find it somewhat comforting to have people around who help keep me tethered to a more rational world.
They can't solve my problem, or stop AW from destroying herself, but at least I have somebody to talk to. As a person who has never really been very social or talkative, this has come as a revelation to me.
Maybe, at the very least, you can give your sister some comfort... a temporary escape from the madness. There may be more you can do, but I'll leave that advice to the experts.
Good luck; be well.