(I edited your letter due to its length but I think I kept the essence of it.) There are plenty of things to worry about here and plenty of things to distract you from the painful truth you are avoiding: The problem isn’t with M, it’s with your boyfriend. Yes, twins have a special bond. Yes, when a person dates a twin, she usually has to accept that she will always have to share her sweetheart with the other important twin-relationship. But what makes this situation different and dangerous is M’s addiction and your boyfriend’s refusal to see it for what it is. Instead of trying to get his brother off drugs, he wants you to make nice and accept it. He may think he is protecting his brother by staying with him but unless he is willing to confront his brother’s ideas about drugs, he is enabling the addiction, and may be contributing to his brother’s eventual death.
Moving in after only a month was very unwise. I think the healthy thing for you to do is to move out and pull way back so you can start over. Give up arguing with M. It’s pointless because he has no interest in hearing what you have to say. Date J if you want and get to know him apart from his twin. If he is able to separate from the drugs and from his brother’s crazier ideas, a relationship with him might be worth another attempt. But if he insists that you join him in enabling his brother’s pursuit of martyrdom for drugs, you need to leave. You won’t be able to live with knowing that you were part of another musician’s death. Meanwhile, stay open to meeting other people and continue pursuing your own goals. Art Therapy is part science, part art, part heart. It’s good work.
I wish you well.