I am assuming you are flying, how long is your flight? In the end things will happen that you could not have planned for, so do not stress trying to think of every little thing. Plus, you will have two other adults helping, big plus. I would bring carriers (wearable ones) to help as you walk from car to flight etc... I would pack a light weight blanket that can be layed out on the floor in case you need to put them down, change a diaper in a hurry etc... You can take your stroller with you all the way to the gate.
I would also suggest using a backpack instead of shoulder diaper bag. Pack your diaper bag for use once you get there.
Anything you might need that you have forgotten can be bought once you land.
You will be fine!