You can't do anything besides voice your opinion on how you think of her. If at the end he still marries her, then you have to be a good sister to her regardless of how much you cannot stand her.
Build a good relationship so that she knows that you as a sister am a good sister whom she can come talk to about her problems in regards to your brother. That way, you will have some influence to her and also make her realize how to work or be a woman that a man wants.
I have five sister in law and trust me, they come and talk to me about all sorts of things. And no, I'm not always on my brother's side, but I have to be a good role model (even though I'm younger than them) so that they will see it through me that I am a good sister why my brothers are the perfect husbands for them...LOL.
I know this all sounds too weird and probably makes no sense...but if you understand somewhat of what I'm trying to explain, then you'll get the idea.