Why Can'T It Be The Same!!!!!

Ok so I've been with this guy for alomos 2years, and engaged. I have 2 girls from a previous relationship and We have been able to maintain a good relationship for the girls sake. I don't get involved with his life and he doesn't get involved with mine either. We both get along with each others current partner, his girlfriend is sweet and caring and i know she cares about my girls and treats them as if they are her own. As long as he's there for his daughter, HEY whatever he does with his life is own ****! :) Now, on another note, my fiances BABY MAMA causes BIG TIME DRAMA!!! :( I mean this ***** is crazy. MY fiance has a 18 month little girl, and shes adorable, I love this little girl. I have never done anything to the baby mama for her to hate me as much as she does. They were over since she was 2 months pregnant. I hate that she puts my fiance thru such misery. Ever sice she found out about me she freaked out and its been one thing after another with her, late phone calls harassing , threatening, police, cps, courts.. etc. We picked up his little girl on her on her bday, to our surprise her head FULL of HEAD LICE!! My fiance was upset, shes only 18 months old and to see her like that broke his heart. We bought her hair treatment shampoo , I sat for hours cleaning her head, the next day we sent her with her mom. My fiance made a comment about it to her and she slapped right across the face and went off on him! I guess she got offened or something. After that everytime we pick her up its the same routine. She always comes back with head lice. Shes always wearing old clothes dirty clothes ( i understand shes a baby but to always have her clothes and face full of stains is beyond me), and never have i once seen this little girl wearing shoes. What can we do about. My fiance and I constantly argue about her because shes always calling him saying this and that calling hima deadbeat father messaging me threatening me , callin my girls the B word.. i mean wth! My girls are 7 &10. They have done nothing to her.I can safely say that my fiance is a wonderful and responsible father, and that fact that he puts his daughter before anyone makes me love him even more! But his baby mama is wearing me out with all this B*******.. Does she not have a life! We got into a fist fight once when we went to go pick her up, she went crazy when she saw me carry her and put her in the carseat, the little girl started crying, the baby mama started yelling and going off on me saying that she doesnt like to be in carseats and that i was being selfish and trying to keep her away from her father in the car.I mean really, is she serious, (considering that the baby mama has her wothout one when she rides in the car). This is only a And anyone who wants to judge me for talking smack about the baby mama and tell me that I shouldnt say anything because i dont unsderstand the situation is WRONG! Im a baby mama myself and i have never caused problems for my ex and his partner! What Can I do