I want to start this off by saying that I really wish it hadn't come down to this. I've forgiven her time after time, and I always try to see the best in her when things have gone south in our relationship. I feel now that it might have been my fault for allowing her to be so involved, but we're in this situation now so we just have to deal with it.
She was my freshman (college) roommate, and helped me get through some of the pain of being sexually assaulted when I felt like I couldn't trust anyone else. For that I'll always feel like I owe her. However, throughout the 3-4 years I've known her, she's been manipulative about getting attention (i.e. continued to lie about being pregnant after she confirmed with her doctor that it was only a hysterical pregnancy, make everything constantly about her even when our friends were going through hard times, etc) and she's lied to me on so many occasions. After each major thing has happened we've fallen out, then become friends months later and were stronger for it (with the promise of her seeking psychological help for personal matters). We ended up being friends again about two months before my fiance proposed, and instantly made the relationship about her. The moment we got engaged she started telling me this tale about her being engaged when we were fallen out, and how her fiance had left her after some car accident. I have never believed it, and feel bad about it, but here we are. Yesterday I found out that the first time she had met my fiance (she had stayed over the night before), she had waited until I got in the shower to go stand over him and wake him up, then threatening to kill him if he hurt me. I'm disgusted and hurt, and now concerned that if I were to ever vent to her about a problem we may be having that she'll do it again.
At this point, I really don't know what to do or how to do it. She has always been incredibly possessive of me and almost even obsessive. I get calls from her at least twice a day, if I cut/color my hair she does too, if I go through some experience she makes a point to act like she did too, and now she's going around telling everyone that we're sisters. She's even acting like she is allowing my fiance to join *OUR* family, and hitting on the best man (also engaged) because she wants to literally be like family. I honestly am lost as to how to handle this. We're a year out from the wedding and this girl is stressing me out tremendously, but I feel like I owe her after everything she helped me get through. I wouldn't even know where to start, truth be told, if I did remove her from the bridal party. Please send help.