Competing with my brother in laws girlfriend?

Ok so I'm really pissed right now so this is probably going to be all mixed up. But anyways heres the story: my fiance and I have been together for 3 years. Getting married in a month. (planning to anyways) We do not live together yet. Anyways. I do everything for him. Anything he needs or wants, I get for him. And I mean anything. He does not appreciate any of this. Instead, his younger brother, who is only 17 has a girlfriend who is older than him and they have been together for about 6 months. Well for some reason my fiance has taken a liking to this girl. I however cannot stand her. We went to high school together, and I have never liked her. There is obviously is a problem with them being "friends" because she talks bad about me to him if he and I are in a fight. Which, usually we only get into arguments if she and his brother are around. Oh and she went and told everyone that she didn't like me, and my boyfriend tried to mess with her head and make her feel bad about that, so she tries to make up with me (like I even want to be her friend) just to make my boyfriend happy. Why the hell is she trying to please him anyway?? He brags about how cool she is and what a nice girl she is to EVERYONE, and this isn't even his girl. His brother doesn't even like her that much, he cheats on her all the time. On the other hand, I never hear about my fiance bragging about me or what I do for him or how I great I am. Never. Instead, today we get into a fight and he gives his phone to his brother, who tells me that we should rethink getting married because we aren't going to last. Huh. So I just asked him to mind his own business, and that was that. Then my fiance told me hes done with me and always wants to leave, but I will never leave him alone. Reallyy?? Just this past week he went to my grandparents to ask for my hand in marraige, and took me to his moms to share the news. Hmm. I don't know what to do. I need something to make him think. Something to scare him I guess, or at least to get him to change these ways.