@SickOfCrap
I get that you feel like shit with the pregnancy and GD, but unfortunately, that's life.
You always knew he was already a Father. And I get that it was shit of him to spring the kids 2 week stay on you with zero warning. But that's a 'you and him and communication' issue not a DC issue or an Ex wife issue 
It's totally unreasonable for you to try to dictate when he's allowed to see his kids because you're tired ffs ! (You think you're tired now ? Just wait until you've had your baby for a couple of months ! Trust me, you're not that tired yet !!)
What his Ex does in her kid free time is bugger all to do with you, but of course the kids should be left with their Dad while she's away.
Did you think that his Ex should have her child free holiday revolve around your pregnancy ?
It sounds like you're not really understanding what it's like to be a 'parent' yet, but once your DC is here, I think you'll understand a lot more.
A good parent will do anything to spare their kids a moments hurt or anguish, and it sounds like he's a good parent.
And you'd better think again if you expect your DC to prioritize your DC over his first 2 kids, because that will never happen, because they are all HIS kids.
He's doing his best for his DC, and you should applaud that, not keep trying to force him to put you above his children.
I hope your pregnancy goes well and you have the birth you hope for. 