Have you ever been so offended?

[Fromper]

If it was me personally, I would write a well-worded, profanity free message pointing out all of her flaws,everything wrong with what she's saying and what a terrible person she is for saying it, and then, in the final sentence, tell her "kindly go fuck yourself now."

If it was me.

I wouldn't tolerate that kind of bullshit from even my closest relative, and once you cross that line, the gloves are off.

That would be my advice. Let her know that you won't tolerate this BS again, or else you WILL here it again.

Your choice....but be prepared for her to do it again. She is not going to change until you tell her off.

She won't change after telling her off, either. But instead of saying these things to Losty's face and pissing her off again, the aunt will just talk about her behind her back. It's highly unlikely she'll change, though. But most can just be trained to pretend to be a better person, even if they're not.