Yes, drug addiction is terrible; almost nothing is as destructive and devastating - both to the person who's addicted, and to those who care about them. Some of the most heartbreaking letters I receive are from people who have become enslaved by drugs - and from their families.
You are probably right; nothing will change for your niece if she keeps on living the way she is right now (unless she gets arrested or destroys herself). Small or halfhearted measures are seldom effective with someone in her condition. This is why I urge you to speak with your doctor or pastor or other knowledgeable professional about the resources that are available in your community to treat drug addiction. Then seek advice on what steps you can take to get her committed to a treatment center.
But the most important thing you can do is encourage your niece to open her heart to Christ and seek His help for her problems. God loves her, and Christ can give her a new reason for living. The path ahead may not be easy - but with Christ's help she can discover the peace and joy she seeks.
In addition, pray for your niece - constantly and fervently. No situation is ever completely hopeless, because God "is able to do immeasurably more than all we ask or imagine" (Ephesians 3:20).