That’s the best advice, I think. Have to plead in earnest with God to intervene!
In addition to church/prayer as recommended, I would recommend you and your wife start attending Al-Anon or Narc-anon meetings and reading their literature. You apparently love your nephew. He is sick, and has a disease related to his drug use. Unless he is an immediate danger to himself or others, involuntary commitment is not going to happen. In fact, one of the worst rehabs is an involuntary rehab.
Your trying to be “helpful” here may actually be a bad thing for your nephew as it deprives him the opportunity to confront his disease. You may have the right, loving intentions but for the wrong situation. It would be helpful to you and possibly for him that you learn how to detach yourself from this situation with love. Your nephew could hit rock bottom and then enter recovery, a lifelong process - there is no cure for this disease. Or he could die. That is the reality - one might feel like one is watching a train wreck in slow motion, and yet denies the reality that if one tries to stop it, one will be crushed by the train wreck. For those of us who are quietly out here, we are in your shoes, we are with you. Find your Al-Anon or narc-Anon and begin your own recovery in how to live with this terrible disease that afflicts your loved one. You need to take care of yourself now. It won’t be easy, but it will be worth it. Those who recommended this to me 8 years ago, for them I am eternally grateful. Good luck.