I would stop replying to her at this point. Your dh has given her an answer, explained why that was his answer and clearly told her that he will not be changing his mind. Continuing to reply to her just gives her an audience for her drama; don't do it.
If it's important to her for her aunt and uncle to meet her kids, she can arrange a get together during her time. Your dh's time with his kids (and your daughter's time with her siblings) are equally important. What a cow to lash out at you and make comments about your family. It's hardly *your* responsibility to pay for the running around it would require for a get together *she* wants them to attend.
She will probably never approve of or accept you and Audrey. The good news is that she doesn't have to. She's free to choose to be miserable for the rest of her life if she wants to be.

Don't even get sucked into having a conversation with her about your love for ALL of the children. You and your dh know that everyone is equally loved and equally provided for, and I'm sure the kids know too. No matter how many reasons you give her, she will never accept what you're saying and it would be a headache you simply don't need.