D.B.
As a stepmother with a very difficult bio mom, I suggest you stop engaging with her. Don't answer her calls or texts. Don't be rude if you cross paths, but don't engage. If she give her opinion, say, "Thanks for your concern. I know you love the kids. I will discuss this with Joe and we will decide together." If she persists, repeat, "Thanks. I'm sure you understand that there are so many people involved, and I will confine my conversations with him. I think it's best for the kids to know that he and I are still their parents, despite the divorce. I'm sure you agree." Then, get off the phone or change the subject if you are in the same room.
Don't discuss her with your ex. Discuss the kids. Always, and only. And superficially. Don't show your irritation. If you don't get involved with her, there's not "side" for him to take. I'm sure he discusses the kids with her, and really, that's fine. If you have someone in your life (now or in the future), you'll discuss with that person as well. But the new person in your life doesn't weigh in with your ex, and you stop weighing in with your ex's wife.
You co-parent with him, and he can talk to whomever he wants. But your conversation is with him, period.You won't do well by dismissing her outright. She's in the picture, like it or not. Your job is just to be cordial to her at things like kids' baseball games and school concerts. That's it. You'll be glad you did, glad you took the high road.
Your kids WILL benefit from a loving stepmother, and from seeing what I hope is a good 2nd marriage with their dad and stepmother. So support that. Period. The less you two are ticked off at each other, the better for you and the better for the kids.