I'm so sorry for what he's putting you and your girls through. It's one thing to deal with the loss of your marriage but quite another to watch your children lose their father. And I fear that his parents are also detaching from the girls too. It's shocking and unfair.
I can see that he is tying you up in knots with his rewriting of history. She " made me " get married / have kids / have another kid and now that's inconvenient to me and I regret my choices I can blame it all on HER. And I can do anything I want because " I was unhappy ".
You have now noticed that he behaved totally differently with DD2s health problems to DD1. So you KNOW it wasn't about her and it's wasn't about you. It was about HIM, he was already detaching from her and from you, probably because this or another woman was on the scene.
This isn't a sudden change that's overcome him, he's not ill and he's not having a breakdown. The problem is his very large sense of entitlement which allows him to walk out his kids when he's got something better to do . And of course because you will always pick up the pieces .
He is more attentive to you now ( than the kids ) because he can feel that you are growing away from him. He wants to keep you sweet because he's cake eating. He needs to keep you on the back burner in case things don't work out with OW, or he changes his mind.
He doesn't need to make the effort with the girls because he can always enforce his legal right to see them, regardless of how badly he has treated them. So he can just drop them until it suits him to see them again.