I know first hand, that a lot of JW youths, when DF'd or "out", sometimes find comfort in pills, and alcohol. We know this is a false hope, though. I have lost a few friends to this scourge. Try to help him, the best you can, sounds like he needs it. Recovery is VERY possible. Options like Methadone, suboxone, detoxing, counceling, they are out there.
Its spine chilling, how unloving his own parents are towards him, and literally watching him die. Once a person succumbs ultimately, to this disease, thru death, you CANNOT turn back time. Please remember this, you only have the chance to save him, while he is alive, and only then. His parents may feel they can "replace" him, but YOU can never replace your nephew. One day, you two will be raised up high together, into a place of worthiness. Your sister, and dick-in-law, may very well be replaced themselves, in their OWN GODS eyes.
I really wish, and probably looking back you do too, that you called the authorities on your Bro- in -law, all those years ago for child abuse on a 2 year old. Thats what should have been done, but you cant go back in time. However, you DO have this time NOW, and armed with the knowledge you have about your nephew, please take advantage of this time. You dont want to wish you could go back. Now you have the chance to help.
I wish you and your nephew the best. You have my love, and support, for whatever thats worth. You seem like a person after my own heart, telling it like it is, and not putting up with bullshit. Dont put up with the bullshit, that the drugs are doing to your nephew. Now is not the time to worry about his asshole JW parents, but instead to worry about him.
Arm yourself with sword and shield, and go slay the dragon that is attacking him. The dragon is sly, and cunning. It is evil, and malicious, and the epitome of fear. You must be wise, and fierce, strong, and determined. Good luck to you, and Godspeed.