Second the poster who has recommended Nacho Kids.
The model here is that you never say anything negative about the kids, the exw, or either parent's parenting. And that is because you don't have to. They are not your (nacho in a Texan accent maybe) kids.
I've been on a real journey with this one. I am now pure Nacho. DP's kids, their contact schedule and their mum's behaviour has nothing to do with me.
The best way to Nacho, in my experience, is to get the father of not-your-kids to stand up to any behaviour that making any of his first family problems your problem. You can expect to have a protective ring of steel around you, your relationship and your time together. In this case, when you are always protected, the exw does not have to be anyone's problem. The man you are with must make it so.
Men in my experience will choose the easiest option with the exw always and then moan to their partner about it, to try and make it the partner's problem when things go wrong. 'She's done this again, what shall we do?'
There is no 'we' in this ideally. As the step parent, all you require is firm boundaries and expectations.
I had to tell me DP we were taking a relationship break in terms of my not being around at all when he had his kids, so that exw antics were absolutely always firmly his own problems.
Left to his own devices he's managed incredibly well. Most of the right boundaries are up now. And I am full Nacho because I can be.
DP played a trump card. He told exw that if she expected to be in control of his contact and NC time, he wasn't really a 50-50 parent and they should drop the 50-50. Sure, she wouldn't be able to work full time anymore, but this was her choice. Real co-parenting or her being the primary parent, taking the appropriate balance of rights and responsibility.
DP did this because I was not around to enable anything otherwise. There was no 'she wants me to do the school run today but i need to work, so can you do it' because I wasn't around.
This is quite a long ramble, sorry, but I really do think the best way to stop yourself having any exw issues is to remember its not your problem and absolutely make it his problem. Nacho Ex Wife would be good follow up series!