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.
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.

I hope progress has been made and everyone will get on the same page to celebrate an 80th Birthday.