Remember: keep your friends close and your enemies even closer. You'll be able to keep a closer eye on her little games and tricks if you're there, so put a big smile on and try and be friendly with her. Don't compliment her or flatter her too much, but tell her how things stand and how nice it is her and your other half are such good mates: "You know, he says you're like the sister he never had!"
By alluding to their relationship as a brother-sister thing, you'll help get rid of any fantasies she's harboring about getting together with him one day. But whatever you do, don't trust her by telling her any secrets or discuss about your relationship - it will come back to haunt you.
What if she's totally unbearable and you can't get on with her? Be dignified, don't have anything to do with her but stay polite. Whatever you do, don't criticize her constantly - he'll only take her side and defend her, either our of loyalty or just for the sake of contradicting you. Don't forget that for him, criticizing her is like criticizing a sister or even himself, because they've been through so much together. And however evil she is, don't ever give him an ultimatum and make him choose between the two of you. "It's her or me" isn't going to work. Relax and tell yourself you're the one he's with, not her.