I didn't read the entire thead... however I know of your situation... from top to bottom...
I honestly can't say anything you will do would end up positive for him, he has to actually want to stop...
Now, your sister calling you crying... etc... I know you feel horrible, and sympathy for her... but who other than herself caused this situation? Yet she wants someone else to fix it?
If you step in right now Trin... and i'm seriously looking from an outsider whom knows not a damn thing going on. You make a statement.. then it trinkles on from that... a bust, he goes to jail... so forth ...
Who is the the bad guy... YOU!!! His mother clearly knows he has a problem yet won't do any damn thing about it because she also has a problem and to do so would mean that she would have to acually straighten her life up. RIGHT?
If i were in your shoes, i would prolly step in... not go the law... but step in, try to talk to him... etc... In a nut shell... this boy knows what he has been around his whole entire life. Show him different and maybe ... just maybe... he will let all that go.
Please call me before you decide to contact anyone... I just honestly don't think you should do anyting right now without actually considering the backlash you will have and ... considering is it actually worth it? Or more worth a focus on you and your kids... You have your own things to deal with. Not that he doesn't deserve or need help... but his mother should own the fuck up and take control... right?