Mi sister might be bringing my ex over?

She might be bringing him by for this family event and I can't stand him as our break up didn't go over well.(she knows I don't like him and aware of the reason) He's a player (which is the main reason I broke up w/ him). She can date who she wants, however, I'm not sure how to stay relaxed, calm and polite when I have to encounter him. How do I keep my composure?