As a general matter, one person is not obligated to report the criminal activity of another--there is no liability for failing to report. That said:1) If you are with your brother when he has illegal drugs, you may end up, depending on the exact circumstances, being charged as an accomplice, co-conspirator, etc.; being with someone who has drugs makes it very easy for the police to arrest you, too. You may ultimately win and be able to defend yourself, if you had nothing to do with it...but you don't want to be in the position of being arrested and charged and having to defend yourself.2) If there are drugs in your home, then if you own the home, it is sometimes the case that the authorities will confiscate your home (civil forfeiture) as being the place used for criminal activity (I used to work at the U.S. Attorney's Office in Manhatten doing this--I've seen people lose their homes and cars because of what a relative did). Or if you're in a rental premises, you could end up being evicted--for example, if your lease has a clause prohibiting criminal activity generally or drug use specifically.In short, while you don't necessarily have to report your brother, he is potentially putting  you at a great deal of risk.