You are not guilty of being in possession of marijuana if you are merely present in a location where it is possessed or used. However, if the home was ever searched you might put yourself in a bad situation. Many prosecutions are based upon circumstantial evidence. Circumstantial evidence is evidence that has other possible inferences. A classic example of circumstantial evidence is when a person walks indoors wearing a raincoat that has tiny drops of water on it. This is circumstantial evidence that it is raining outside. Could the police circumstantially believe that you were also in possession of your roommates marijuana? I don't know. With drug charges possession does not mean ownership. The prosecution does not have to prove ownership. Possession can be joint (no pun intended). Joint possession means that more than one person has the right to possess the drug.
If you want to eliminate any possible risk of being prosecuted, move out. If you want to continue living with your cousin and reduce the risk of being prosecuted in the future, make sure that he keeps his marijuana and any use paraphernalia (bongs, rolling papers, etc) in his own room, and not in your room or any common areas of the house.
I agree with Attorney Hilf's answer. The police won't automatically assume you don't have anything to do with the controlled substances. The law requires more than mere presence, but it is all fact sensitive. You could get erroneously implicated, so use good sense and take steps to distance yourself from any illegal activity.
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Don't let him use your car.
He could be into selling it and that would be more trouble.
Find a new roommate.
Expect Law Enforcement to search your residence one day soon and anything YOU have that is illegal will get caught up in it.
Curt Harrington Patent & Tax Law Attorney Certified Tax Specialist by the California Board of Legal Specialization PATENTAX.COM This communication is general information and not legal advice, and does not create an attorney-client relationship. This communication should not be relied upon as any type of legal advice. Please note that no attorney-client relationship exists between the sender and the recipient of this message in the absence of either (1) a signed fee contract and (2) remission of an agreed-upon retainer. Absent such an agreement and retainer, I am not engaged by you as an attorney, nor is any other member of my law firm.