I hope that I'm not too late in my response. I agree wholeheartedly with the others. Screw the party! From what I hear, you are not in the space right now to be in a place that doesn't feel safe to you.
Why make up a lame excuse? Simply say, I'm sorry, but I won't be able to attend. If she pushes for a reason, just say, that it's personal. If she keeps pushing let her know, I appreciate your concern, but let's just drop the subject.
There is a time and place for everything and even though you may want to know more from this relative, the risks FAR outweight the benefits at this point in your life. You need to concentrate on making Jasper whole and comfortable. AND I'm willing to bet that Andy is more than willing to field phone calls for you - there have been times that I've asked my wife to open mail, answer the phone, etc for me because I simply couldn't handle it. She happily did so because she knew that it was too painful for me to deal with it.
Finally, don't feel guilty that you didn't go.... easier said than done. However, you need to keep reminding yourself that you and your life partner are the two most important people in your life right now. Andy is going to help you thru this and the only thing to remember is to thank him all the time for his support because he's probably "winging it" as to what to do to help.
There are no unresolved issues - they just didn't resolve themselves the way we would have liked. "Grinder and Bandler - Neuro-Linguistic Programming"