What should I do?

My situation is, my brother and his girlfriend moved into the house I am renting and they are smoking weed (it is still and illegal drug in Ontario, Canada) in the house at times when I do have my two year old child and when i do not have my child. How it goes is that i get my child literally 50% if the time. I have been living in my parents second house as a renter for 11 months now. My brother decided he needs a place to live with his girlfriend and moved into this house. We are currently renovating the basement in which they where meant to reside, but the reno is ongoing so they are currently in my son's room, while my son is in mine. My brother insists that because he is paying rent and the basement reno is incomplete, which he is in no hurry to complete, that he has all rights as a tenant to smoke weed in the house regardless when i have my son. My parents have told him and his girlfriend a few times to not smoke it in the house but they disregard what was been said.

Should I call the police on them the next time they smoke it in the house. Should I take the risk of Childrens Aid Society being called in because the illegal substance is being used in the residence of a child? The last thing I need is CAS being called, but I do not know what else to do. My brother is not listening to me when I say to him that i cannot have it smoked in the house.