Heh, I thought my brother was bad, but he's nothing compared your sister...
I for one wouldn't "help" her. I tried it with my brother (long story), and it simply doesn't work. You only end up shielding her from the consequences of her actions, allowing her to go on with whatever she's doing wrong. Eventually you have to accept that she's an adult, and is capable of making or breaking her own life. Nothing you do will change that. She has to decide to change herself; you can't do that for her.
...but I'm sure glad I'm not in your shoes.
