After the kids came home the smear campaign kicked back into high gear and now I feel like I have possibly made the kids not like her.
The main question is how do you tell your children whom you know are on the right track not to worry about it. I don't want them growing up to think this behavior is normal and I don't want to have to defend myself, possibly in court.
There is a really, really good chance that your kids don't like your ex because she is verbally abusive and mentally ill, not because you validate them. Validating how they feel is not going to make them feel something they don't feel, it's going to make them feel that
you get how they feel. What are you saying to your kids when they talk about what happens at mom's? Maybe we can help see if there are some phrases that will validate them without adding fuel to their anger. Also, 15 can be a really hard time for girls. Your D has the added challenges of a parent who is mentally ill. She may need some help sorting through grief and other difficult feelings under that anger -- usually anger is a secondary emotion. If we don't deal with that when we're young, it snowballs and creates all kinds of problems in intimate relationships later in life.
I'm guessing when you say that you don't want to defend yourself, you mean to false allegations of parental alienation? It's probably worth documenting everything that happens. Keep a journal about how your kids seem after they've been at their moms, the smear campaign.
There are also some good resources about parental alienation, including an article about the legal definitions for it. And then consult with an attorney -- you don't have to retain one, just pay $100-$200 for 30 min to ask what you can do to protect yourself. Dealing with Parental Alienation:
https://bpdfamily.com/message_board/index.php?topic=182254.msg1331467#msg1331467Therapy for the kids is a great idea. If your ex won't agree, you can contact the school counselor or family counselor and ask them for recommendations in the community, and then just take the kids. I have never heard of a case here where a judge disagrees with therapy for kids, even when both parents are supposed to be involved. If your D15 decides she doesn't want to go to her mom's, and you take her to see a T about that, it will be hard to prove that there is any PA going on.
Also, it's telling that your D feels so protective of her little brother. In my family, it was as though my brother and I were pitted against each, which is apparently common in families with PDs. Your D is allying with her brother, and that's a good sign.