Kids will be kids...
I am the first to admit that in the presence of my two sibblings my maturity levels reverts back to somewhere between the ages of 9 and 13. I have noticed that this is especially true for me and my sibblings when we are squished into a confined space.
A few years ago we had to share the back seat of a car and it started happening... each turn the car took my brother exaggerated it with his entire body, totally annoying me in the middle and my sister reverted back to being especially whiny (or was that me).
I wonder if when we are in our 60's the three of us will revert back to being kids when we find ourselves in a confined space together... I hope so. Its much more fun that way!