Dated my friend's brother, he blew me off, now I have to see him again. How should I act? help please :/?

[Cynthia]

So I've been friends with her a few years but never close until recently. I had heard her talk about her brother before but finally met him at a gathering she had. We hooked up that night and started seeing each other. We spent every weekend at each other's places, he met my friends and sisters, celebrated my birthday with me... Then after a month, I noticed he was taking to me less and making excuses for why he couldn't see me (he lives an hour from me so weekends were the only time we could spend together) after a little bit of snooping, I found out he had started talking to another girl.
I stopped texting him, and he didn't text me. We just stopped talking, just like that. Still followed/liked photos on Instagram but then I unfollowed him once he started posting pictures about that new girl (he realized a week later and unfollowed me) but we are still Facebook friends. There was never any confrontation, I never said anything against him or made it "drama." Problem is, my friend is having a Christmas party and I'm going to have to see him. I don't know how to act. He did blow me off, but I don't want it to be awkward or drama since we'll have to continue seeing each other as long as I'm friends with his sister. Help! Do I ignore him? Play it cool? Be nice?

[ssff]

just ignore him, sounds like a gutless as**ole

[ssff]

Play it cool like you don't care that he blew you off at all. Like you had another line of guys waiting to hit on you the entire time. Don't show him that he hurt your feelings at all, it's just going to feed his ego and make it so that things are way more awkward.