No - I had these children with him, not his whore.
I would be waaaay creeped out if my new husband cc'd me on comms about his kids with his X. I'd want to know - yes so maybe a BC but copying me in? IMO that's tacky as bro.
If my SO asked to be cc'd in emails to the sad clown I might agree to BC just to keep him in the loop but not CC because it's tacky even though he won't be an OM.
Now if he happened to remarry a decent human being who became a part of my girls lives I probably wouldn't mind interacting with her - I still don't think I'd be parenting in a triangle. I didn't want three people in M - I sure as shit don't want three people in D.
I'd completely ignore the CC as it clearly shows the level of trust between them. If she doesn't trust him with an XW who knows what he is imagine the leash she has over his other interractions? See it as just that, a leash - on him. NOT you.
He may be doing it to yank your chain but it's more likely he is doing it for her benefit - whatever the hell that might be.
She'll soon learn that no matter how much you watch them these parasites always find a way to act on their Wayward thinking.