Don't give your sister's addiction any quarter whatsoever. If you need to call the police do it. You may be saving her life.
I called the police on my dad when he was in a drunken rage and had threatened to shoot my mom and I. I knew he wouldn't do it, but we were both fed up so I did what I had to do. The police came, my dad got belligerent and tried to bully the cop and got arrested. I had to watch him get cuffed and stuffed right outside the front door of his own house while his pants were falling down around his ankles. He was swearing at me and calling me (his only child) every foul thing in the book. Yes, it hurt. But when he was gone, my mom was safe. When he woke up the next morning in the county jail and didn't know how or why he'd gotten there, he knew he had a serious problem that needed to be addressed. That was his bottom. He's been in recovery now for 5 years. Do what you need to do.