If your roommate is committing a crime, even if he is your brother-in-law, then yes, you can have him arrested. If he has recently been through the criminal justice system, he may still be on probation, and if he is smoking marijuana, he would certainly be in violation. A call to his probation officer may be all you need to do.
Most leases hold you are responsible for the guests you allow into your leased premises, and if they commit crimes while in your home, then you can be found in violation of your lease. Should the landlord find out, he could terminate your lease, and require you all to move out.
Since you are now aware of a problem, it is up to you to take some action. Calling the police will not necessarily get you into trouble, but if the police are called by, say, a neighbor who smells marijuana, then that may impact negatively your tenancy.
I hope this answer has been of some help to you, and I wish you the best of luck.
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