I'm so sorry you're trapped in this stressful fucked up situation.

And you are really doing a good thing by handling this calmly, asking to talk openly, rather than reacting with anger even though you probably just want to never see her again. It shows how much you care for your fiancee.
I know this type of person, who can't/won't control her manipulative impulses and then denies that anything has been going on when their shit hits the fan all over everyone. It sounds like she's insecure. Needing a back-up man to give her attention and validation when hers is on the outs. Do you think she sees you as a threat (like females sometimes do...) and wants to keep you feeling uncomfortable and unwanted?
I hope your fiancee sees where you're coming from, but based on the intensity of the argument, maybe that's not the case. Hopefully the two of you calling her out in person will be enough to break her denial down. Does her man know about her behavior with your fiancee when they were living together? Discussing that with him might help your case.
It sounds like she has been fucking up all three of y'all's shit. Not to suggest conspiring against her or backing her into a corner, but maybe if the three of you speak up together about her childish behavior, she might break down? Who knows if she will actually change, but if you can at least clarify and point out what she is doing to you all, it might be a lot easier to handle.
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