A while back I posted a video of Alice laughing at her brother.
In the clip, Alice only responds to Elliot with laughter — her parents get no reaction.
BuboBlog Westchester County correspondent Kate noticed a similar phenomenon with her kids, so she asked their pediatrician about it.
Apparently it's a survival mechanism. The baby knows the parents won't hurt her, but the older brother is a bit of a wild card. So the laughter is an attempt to butter him up — not unlike how you'd laugh at your boss' lame jokes (except your boss probably won't smother you in your sleep).
It must be working because Elliot adores his sister. Way to kiss up, Alice!