I'm a pro at this.
Keep the visit shortish.
Ie fri eve to sun lunch.
Break it into blocks of time.
Take oldest of youngest out to some event (party or whatever) and leave other with DH and MIL. Soft play also works- you and 1 child, dh and mil man mark rhe other.
You prep all the meals and wash up as keeps you out of the way....
Go to a gym class / church / fruit and veg stall on your own one of the mornings.
Have some event Sunday PM from about 2 so its a natural "close" to the visit. I often just book something and say its been in the diary ages....
Deep breaths and offer to make tea when your dh announces he needs to play on his phone for 30mins go to the toilet.
Good luck.
If you find yourself alone grey rock. Answer questions minimally amd dont make small talk.
I'd generally keep my oldest with me to minimise her bullshit and id shut down any inappropriate chat.
Might be worth taking both kids out and leaving dh and hus kother to it
Pre kids id just go out all day as I had something that "clashed".
Next time send him to her and you stay home with the kids... I wouldnt want her near them tbh.