Wow this is a tough one because at 18 he is almost an adult and diciplining him may not be an option depending upon what the relationship between the two of is like.
I have an 18 year old ss and he is in college. He pretty much does what he wants but at this point in life the choices are all his. Fortunately for us he is pretty honest and open with us and will come to us for advice but as I said, the choices are his to make.
I wish you lots of luck with your brother. If you don't mind me asking...why are you raising him?