WOW...What a terrible situation..Well, from everything you said, it seems he is a addict, and has been probably for some time...ecspecially with the car thing, and his brother stealing etc..
Does your sister know about your addiction?? If i were you, I would confront him without your sister around, and i would not tell him about looking in his car...I would just be straight up with him, and tell him you know , then ask him if he wants help?? Are you willing to share your situation with him?