Agreed. As someone without their own family to balance things out (same for me) OP could very easily be consumed by her DP’s family.
For some this might sound nice but in reality it’s overwhelming to be the odd one out when all their family dynamics come into play.
It’s easier to manage when visits are spread out. Day to day / every week is a whole different kettle of fish. It’s easy to become a scapegoat or the difficult one because you don’t want to do things their way and your DP reverts back to how he was as a child around his parents.
On top of that they’re literally saying “You don’t have family, we’ll slot into those positions” which I can’t even describe how I would feel about. Not good or comfortable, that’s for sure.
I think you have to take it day by day OP, though I imagine you want it sorted concretely so the anxiety goes away. I can’t see how you can lay down expectations without them being very offended or hurt, because they won’t be able to relate to where you’re coming from.
You need to trust yourself that moment to moment you’ll stand your ground. So if you want a quiet weekend you say that when it needs saying. If you don’t want to get involved in something you speak up straight away. If you can’t be arsed with hosting you tell DP to have fun but you’re off for a swim. Etc.
I would also make a list of positive things about the move. Your DCs will be very happy, there’s a bit of childcare on offer.. it’s not all bad. You’ll have a nicer time with them if you can make peace with it and trust you’ll be able to handle things.