A couple years ago, I moved into a very expensive housing market for a new job. I found a really great house – lots of space, great location, affordable rent. There were two roommates; I'll call them Cindy and Brad. Cindy soon moved out to go back to school, so Brad and I found a new roommate, who I'll call Molly [29/F].

Overall, things were good for the next two years. However, the issues have started to mount. First, I work regular office hours so need to be asleep by 11pm; Molly works odd and irregular shifts so is often up much later, moving around the house. Molly smokes cigarettes and does blow, both of which I've struggled with and prefer to avoid having around. Molly is also pretty messy, whereas I'm just shy of being a full-blown neat freak. Finally, Molly's partner has, for the last 2 months, been effectively living here rent-free. He has a key, gets mail delivered here, and stays here almost every night. Now, he's a really really good guy – does more for the house than Molly does – so it hasn't been a huge issue. However, our landlady has been suspicious and has definitely been lied to about the nature of the arrangement.

Meanwhile, Brad, the master tenant, is moving out to live with his girlfriend. He's a good guy but pretty lazy and disorganized, so is often late to pay rent. I've often had to step in to play the role of master tenant or take on other home improvement projects, so have developed a positive report with the landlady.

With Brad moving out, Molly wants her partner to take his room. And while I love them both as people, I don't want to live with them. Besides what I listed above, they are in a polyamorous relationship and have other partners they both share. I fear that this would tilt the balance from the house being a relatively relaxed environment to being the headquarters for their poly squad. There would be much more of them doing drugs in the living room on weeknights, coming and going at late hours, dumping their stuff everywhere, and generally setting the tone for the house with me as third wheel. I also don't feel like I could just say "no" to Molly's partner moving in, given that he effectively lives here already.

However, I really don't want to move out! I can't afford to live alone, and I don't want to have to go through the process of finding a new house and roommates. I'm really unlikely to find a space that's comparable in terms of cost, location, and amenities. I also feel like I have more of a right to stay here than they do – I lived here first, I have a relationship with the landlady and will likely become the new master tenant, and I doubt she would even let them sign the lease (given that they both have irregular, unstable income). But I don't want to snitch about the current situation to the landlady, or have them feel forced out – I really do love them both a lot as people! I'm also not confident about the ability to set house rules that work for both of us – previous efforts to set cleaning schedules have been ignored, and I'm also outvoted – the rules would also be more for my needs and preferences than theirs, so risks putting me in a position of being a dictator around enforcement otherwise those agreements would get ignored.

Is there a way to make this work, or, at the end of the day, is it my responsibility to move out if I'm not happy here? Or, is there an ethical way to assert what I view as my "right" to this space?