Our girls will be 12 on 9/2. I think I need that book, I might have dropped them a time or two (or it could just be the tween years).
Seriously, they are good kids and it has been interesting over the years as people ask them if they are identical when one is 6 inches taller than the other, do not look anything alike, and have completely different personalities. Enjoy as many laughs as you can.
One of my fondest memories is our first trip to WDW with them when they turned 4 (birthday trip). Every night they would fall asleep on the bus back to POR (sitting, leaning or standing) and I would carry them both back to the room at POR, while my wife pushed the stroller with all the bags of the day in it.
Enjoy the memories. Stock up on the sleep now though.