OP, I would say that she gaslighted you. (I won't get into what the brother said, b/c that was rude in a public setting, but the greater issue here is Mrs. Donny denying the content of many previous conversations.)
I know someone like this. Things would come up in conversation....all part of the 'getting to know you' thing. You remember things about people that might come up in future conversations. This type of thing has happened more than once with her. Sure, they were minor things, but when you're told one thing, and then the person in question does an about-face and denies what she told you previously, it makes me kinda mad even if the things are minor. I mean, for something SO minor, why not just tell the truth? I mean, one time one of her things was that she couldn't wear flip-flops that go between the toes. She went on and on about it, in great detail, about how she couldn't wear that style and it was so hard to find the other kinds of shoes and how she wished she could find more in that style, etc. So then guess what? I see her wearing flip flops that go between her toes. I never said anything b/c it really wasn't worth it, but it was just another case of me realizing that I couldn't believe what she said. There were other times when it was a similar situation to what was in the OP, where there were other people involved, and I was made to look like a fool.
If I was the OP, I would be very tempted to play dumb and ask Mrs Donny about her plans for the party, how she came up with the idea, etc. Then, if she makes any comments about her being from that area, then I would act confused, b/c she told me point blank in front of my brother that she was NOT from that country. Let her squirm her way out of it. Of course, I couldn't do it at the party, but believe me, I would want some sort of explanation for her making me look like a liar and feel like fool. And I would also be tempted to turn it around on her, and ask HER why SHE lied, because obviously she did, because she told you two dramatically different stories at different points in time.
I would really steer clear of her.