I would tell her family. Then, separate yourself from this relationship, as hard as it may seem right now. What you have to realize is there are two parts to an addict: the addiction and the person they truly are. In her case, the addiction has taken over. The person you knew and loved is gone. You can't have a relationship with an addict. They will lie, cheat, steal, basically do anything to keep their addiction going. Do you really want that in your life? You deserve better. Everybody does.
The only thing they love is their addiction. However, it's not really love, it's more like they're chained to a monster. Just recently, I read a Time Magazine article from 1965 about two heroin addicts in a relationship. The article described their addiction as: "It is their jealous lover, and their wrathful God." Sad, but true.
I'm hurting too right now. I know what you're feeling.