So far, you're doing great. I encourage you to keep on that way. Good for you! (pat pat pat on the back)
Here's what I want you to consider, and remember that I am 55 years old and have seen the long-term ramifications of these kinds of things.
I know of two couples where the parents were adamantly opposed to the respective marriages. Both the couples have siblings.
Fast forward a "few" (ho ho) years. Yes, at my age 30 years feels like a "few" years. I was 25 thirty years ago, a time in life when many of my contemporaries were getting married.
Of the two sets of parents involved, only one person is still alive. One of the mothers is still here; the father and both mom and dad in the other set are dearly departed.
What about the marriages? Both marriages are intact. One going on 28 years, the other 41 years.
Now, consider the position of the siblings. They know that their respective parents were not happy with the marriages. On the other hand, today in 2010, the siblings are the only family left, with the exception of a very elderly mother who is in early stages of dementia. The siblings are the family.
How about you? Imagine yourself 30 years in the future. Your parents might have died, by then (I am really sad to be writing these words, believe me), and you and your brother are the only two remaining members of your original nuclear family. Your BFF is long departed, moved to Paris with her rich husband and you've not heard a peep from her in 20 years. (This happens.) You see your cousin occasionally, but they've moved away.
Who's left from your childhood and young adulthood? Brother and his wife. Yep, the girlfriend you dislike now. She's the mother of your nieces and nephews and aunt to your children. With your luck, she'll be their FAV aunt.
Best to try, today, to get on a good footing with her. If Brother ditches her, then great, you've got nothing to lose. Right now your best course of action is to be as nice as possible and keep your negative thoughts to yourself.
I'd go ahead and be FB friends. Sure, I understand your reluctance. But you might as well. You can always de-friend her if Brother dumps her.
Please believe me on this. I've seen it again and again over the years. The "match made in heaven" couple divorces two years later. The "what do they see in each other" couple stays married 30, 40 years.