Teenagers (especially troubled ones) can be the pits sometimes. She's doing a great job at deflecting her mom's flak off her and onto you.
I think communication with the mom about what responsibilities should be taken in the household are in order. From there, stick to those responsibilities and leave the daughter's upbringing to her mother. Walk away from trying to help the girl and keep to yourself in that area.
If the daughter is doing something illegal and self harming, you can counter with "you have a choice, either you tell your mom or I will". If she refuses, then you can let the mom know very quietly, and without judgement let her know whats going on and walk away for mom to deal with it the way she sees fit. That should be the end of your repsonsibility towards her daughter.
Granted you made a poor decision regarding the pot, but it's a lesson learned. Don't beat yourself up over it, but move on from it.
In the meantime, maybe you should begin looking into other housing arrangements. There are always other options to choose from, just a matter of opening your mind to the possibilities and doing your research
Wishing you well curley!

sabby