Yeah, OP is falling into a trap many of us do.
It's easy to say, "they're just that way--leave them be" but, OP, I get that in the course of your life it wasn't worth it, to you, to argue with your aunt but you're no longer arguing for just how she treats you. You've got a kid--a kid who won't understand why she's made to not feel like family.
I would write her an email or something along those lines and I would, admittedly, start with an apology for how things got out of hand. (of course this would come after I've talked with my husband and explained to him my plan and had him agree on it.)

But then, you need to make it publicly and respectfully known that your aunt's behavior was unacceptable and won't be tolerated. If your daughter is made to feel like anything less than family then they can have their wish and not be a part of your family's life at all. Anyone who agrees can join her and their presence will not be tolerated at any family function.
I'd rather have small, loving family functions for me and my family than a huge one where my kids are made to feel any less loved or accepted.