Treatment for a relative

I just caught my brother in law smoking crack in our garage. My sister has already thrown him out of the house once before  for his drugs and because he was bringing it into the house and around their two young kids who are only 14 months and 4 years old. 

He begged me not to tell his wife. He told me he was trying to quit and he was just having a hard time, but I am not sure if I should believe him.

He is a really great guy that I like a lot and that was very good to my sister before the drugs got bad about a year ago. He swore to me if I didn't say anything he would start going to NA meetings and find a treatment program this week.

I don't know if I am being foolish to believe him or if I finally have some leverage to help him?

What I want to know, is , is this a stupid idea? If i force him into a treatment program with this threat of my telling his wife hanging over his head is it going to do him any good, or am I just putting my nephews at risk for no reason?