Why is everyone trying to force me to be nice to my brother's fiancee?

It's getting really annoying! I have my reasons for not liking her and yes, they are good! First of all, she has no real career, I mean she's like 25 and still thinks she can be a model and just works at the MAC counter in the mall part time. Plus she's kind of a bimbo and not to mention the fact that she's very very fake! She always fake smiles and acts like she's nice around my parents when I know she's being fake and I can see right through her! My brother hasn't even been dating her that long and they are talking about marriage. I honestly think he should get back together with this other girl who I am friends with. I mean they were a great couple for like 5 years and she has a college degree and a great career and she's also an all around great person unlike the skank he is with now! I can't help but messing with him about her right in front of his fiancee and "accidentally" telling her what a great couple they were and how we always thought they would get married. My mom gets mad when I do that but I'm 22 years old and I'm entitled to my own opinions plus my boyfriend who is my brother's best friend wants me to lay off her too since "after all we are going to be related soon" which makes me nauseous! I mean seriously why am I the only one who sees through this girl?