My sister married my first love

My sister started acting shady about a year ago. A few months later, I found out she had been seeing the first guy I ever loved. When I confronted her about it, she lied to me and denied it. She was also cheating on her boyfriend of 8 years with this guy! Well, needless to say, it's been a very hurtful and awkward situation and we don't really speak anymore. In fact, she's pretty much disowned our whole family. Anyways, tomorrow our family is having a big event, which I have helped plan & have been looking forward to for months. I just found out my sister and her new husband, who is also my first love, are going to be there. I must add that I am happily married, but I'm dreading this so much!

Just needed to vent a little. Any advice on how to handle the situation would be appreciated, though.