I work in that area. So I may have a different view of domestic cases. The way the law and policy is in my state, MD, a third party who knows what happened can report Domestic Violence and police are obligated to go and check on the victim. They would go to the home, take a report, photograph her injuries, if any, and get her off by herself to ask her privately if she is being abused.
If they believe that she is being abused (a third party report and some type of injury would be enough, even if she denies. Her report or an eye witness even with no visible injuries is also enough.) they are obligated to make an arrest. Even for minor cases. With or without the cooperation of your sister. I don't think that there is anything wrong with reporting domestic violence, because if she is being abused she may be afraid to call for herself.
That happens. So I'd probably call at least once, for any woman I thought was being abused, even if she did abuse drugs. But I wouldn't get involved in trying to figure out what/how much/where they used drugs. And I wouldn't take her in if she was in active addiction. To a domestic violence shelter is where she would have to go. I hope it works out for you.
Love,
KJ