It seems to me that meeting "your brother's long lost daughter" for the first time, IS a big deal and it WOULD take over the theme of the 80th birthday party. Can you imagine how exciting that would be? I think her brother should have had a party for his daughter, long before now, but he hasn't.Maybe I missed something, but I didn't see where your brother said anything about it being an intro party for his daughter - he simply said that he was bringing her. So what's the problem if you have known about his daughter for a year now? Just because you haven't previously met her shouldn't be any reason for you or anyone else who is attending, to feel obligated to make a fuss over her. It will only be an issue if you make it an issue.
I hope progress has been made and everyone will get on the same page to celebrate an 80th Birthday.
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