I was invited to the birthday party of my extended family member (who is a real friend) but there's an outside chance I might see someone I really....

don't want to see in person. Bad history and I am a bit worried they might get drunk and cause a scene. I don't want to tell my family member anything about what it's about... and they're staying at my place so I can't pretend to be sick or anything to miss it. what would you do?