You sound exhausted, time to tell ALL your kids that you’re done being their go to childcare now. You’ll fill in on an emergency and have normal grandparent visits but that’s it. Give them a date, say you’ll keep going until your holiday but will not be doing it upon your return. They’ve got to sort care out for while your away after all, so they need to sort it out to continue afterwards too. I’m sure they’ll all moan and try to guilt trip you but stand strong, say you’re just exhausted, it’s become too full on for you to continue, and with a new baby coming into the mix, and dil going to work more, enough is enough. They will go on and on and on. Say it once to each of them and when they persist just say I’ve already explained why we can’t continue, nothing has changed and I’m not prepared to discuss this anymore. And don’t!! They start again a simple no, not talking about this, I’ve already said. And just keep repeating it. But you and DH must be on the same page and stand together on this.