Don’t be on audition for people who already don’t like you. Which means don’t talk to her. Why bother?

When she met you (and it could have been anyone), she knew that the party’s over concerning her brother. I doubt any other person in her life would allow her to treat them that way, so of course all her ire is taken out on you. And guess what, her parents already know that that is how she is. If she took offence at what some passing joking comment online, she would take offence at anything.

What you do is see the family on holidays. Be super polite to her, no matter what she says or does. Then you will have done “your part”. She won’t do hers, but everyone knows that already. No worries.