god damn dude, just reading your story I wish you could just beat the fuck out of that guy and get your sister out, but it obviously wouldn't work...
This is also a good post
Except for the part where this guy is clearly a meth cook/drug dealer. Scum.
Y'know - What I would suggest is *not* outreach to your sister.
I firmly believe the situation to this Problem lies in dealing with Him. The first course of action needs to be removing him from the picture - Take him away from her, not the other way around. My first suggestion in this matter, of course, would be to do everything in your power to see that he ends up in prison or at least jail - Present a case against him, he's practically dug his own legal grave with how many times he's been arrested for cooking meth, as well as that unlicensed gun (Beware, for obvious reasons). If that plan of action doesn't work, and I really hope it doesn't have to go that far - Interpret my previous words in whichever way you choose to - Preferably ONLY if you are forced.
EDIT: Let me clarify, before someone interprets this darkly:
You need to scare the shit out of him. Break him down psychologically. Make him feel that doing anything to your sister will be the end of him. Make him never want to go near her or your family again. He himself must not feel safe continuing this relationship. Then, once HE pushes away from her, that is when you start outreach. Granted, this guy is a meth cook/dealer, and they are well known to be absolutely batshit crazy, so very much easier said than done.
This is also a good post