Thread:AnimationFan15/@comment-26212090-20161209201520/@comment-24364245-20161209204043

I think Luna should realize it herself. This is how I imagine the scene:

As the six sisters laugh at Lincoln, Luna sees Leni, Lucy, Lana, and Lily not laughing. Realizing that they feel sorry for him, Luna realizes that she's laughing at Lincoln's pain, and immediately stops laughing, and begins to question whether or not she really is one of Lincoln's kindest sisters.