Help!!!

I am 34+4 weeks pregnant. I am engaged to be married and have been with my fiancé for five years, engaged for 2. We live with his parents, temporarily until we find a place of our own. 
His sister, who is also pregnant lives there, but is not planning on leaving once her baby is here. She's 29 weeks. 
The sister smokes weed openly around the house which I am not comfortable with, and the mum and sister both smoke cigarettes around the house too, which I hate also. My fiancé has asked them to stop but they don't listen or care about the possible outcome. 
We are currently desperately looking for another place to live but we simply cannot afford it. What should I do about the use of drugs in front of my baby as a mother who cares but also a daughter/sister in law who lives in another persons house temporarily? 
Tricky situation, but surely our wishes are understandable...