She is not going to stop until you are out of the picture. Just because she is not pitching bitch fits and temper tantrums does not mean anything. It just means she realised that her juvenile behaviour was not only pushing you away, but it was pushing the boyfriend away. She wants you out of the picture. You may have opened your home to her, but she can move out. She is wreaking havoc and causing drama every way possible.
She has made it clear that she does not want to be in a triad. That has been known since the beginning. She just changed her tactic and is waiting for you and your boyfriend to fall apart. Let her go. You are in a constant state of misery and fake happiness. She is controlling the situation like a puppet master. You express your feelings, and she gets mad and thinks you are attacking her. Your boyfriend wants to be with you while you are sick, and suddenly, he cannot handle two girlfriends. She is immature and childish. This is her first relationship, and it is clear that she is not cut of the poly cloth.
If you want my honest opinion, none of you are ready to be in a poly relationship, yet. Where is the balance? That is not to say that you should not open up later, but this situation right here is ridiculous. Somebody needs to put their foot down and end the madness.