Should I believe a crackhead who says he'll quit?

How is he going to enter a treatment program without his wife finding out he was using? She is going to find out. If she finds out you knew he was using and said nothing, especially if his continued using gets someone hurt, she will have a hard time forgiving you. 

His best chance is to tell her now and get into treatment. If he tells her now and enters a detox, he has a chance of saving his marriage and connection to his kids. If he waits for her to find out another way or from someone else, it will be worse for him. Give him 10 minutes to tell his wife and leave for the detox. If he doesn't do it, let your sister know what you discovered.

His addiction will cause him to say just about anything to avoid giving it up, and both of you would be foolish to fall for these "I'll quit tomorrow" stories as anything but stall tactics. You may want to believe them, and he may sincerely believe them too, but all addicts say these things and then, the next day, they can't help going out and using again and trying to get away with it.

Help your brother-in-law get into treatment today. Sit down with him to make the phone calls and tell your sister. You don't have the luxury of waiting. If you wait, the next time he goes out to score, which will be tonight or tomorrow, it may be the last act in the desperate life of a crackhead in cracktown. Pray the kids aren't in the backseat.