Regardless is this event is your sister's wedding or not, you have a serious problem that needs to be addressed if you expect to have even a half happy life. A disorder that makes it so you cannot go to your own sister's wedding is a serious matter that requires either and or medication or psychological counseling.
Having a disorder that paralyzes your social and family life is a horrible way to live. The worst part of it is that you can get help to overcome this disorder....IF YOU WANT to lead a happy normal life. Otherwise, hold yourself up and away from all the joys in life and lead an incredibly lonely and dismal lifestyle.
Okay, you are afraid of seeing family and friends. Why not get on some medication so you can at least show up, see her get married and maybe spend 2.5 minutes at her reception and then leave.
Your disorder is yours, but it also effects other people. Without help, you face such a terrible life as everyone around you grows and advances in their social life, their love life and their careers, while you sit back and see everything you are missing out on.
You are not some helpless victim, you know. There IS HELP out there, you just have to want it. And, if you decide not to want it, then so be it. Just tell everyone to stop inviting you places because you are too anxious to leave the house for a good family party.