Pay close attention to the son.
Does it appear he's trying to suppress laughing?
Does it appear he looks to someone off camera telling him to 'cut it out' and he does?
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Yes the brother is trying to suppress laughter.
Check this out.
Look at the 40 sec. into vid. Dad mouths the words the mom is saying. Scripted for sure. Looks to me like son wants to bust out laughing.
Note the blink rate of the kid’s eyes.
Faster blinking = more fibs/lies