Wow! What a difficult position to be in!

Who's responsible for enforcing the lease agreement? I'd go to them about the sister who's violating the lease and see if I could have them booted from the premises.

That would solve one problem, and perhaps it will even send a strong enough message to the others that inappropriate behaviors and disrespect simply won't be tolerated.

It's horrible to be living somewhere that puts your physical and emotional health in jeopardy, I wish you the best in resolving some of the distress this has put you in, and the sooner you can do it, the better it will be for you!