He will do anything to make her laugh. A smile simply will not do. Forget the embarrassment that a child his age typically feels just walking around in their own skin, he does not care about that. He is willing to; strike a pose, make a voice, contort his face, put on a show, going to great lengths to change her mood. All he cares about is knowing she is happy, knowing whatever he is doing has put that "can't catch your breath" roar of laughter inside of her. That is the kind of brother he is.
Their happiness is deeply intertwined within each other, as if it physically pains one to see the other sad. In his world a smile is always a choice. In her world sometimes a smile is just too difficult to muster up. At times he has physically taken his hands and turned her mouth into a smile. This in turn has created a real true smile, problem solved.
Every night he kisses her forehead in her sleep. Somehow she knows it is him, a smile again. Nobody buys more time with smiles like he does. No one else lights up her face like he does. If we could put some of his love in a bottle for her, we would surely have found the cure for any sadness this life could create.
He is her big brother, he is her everything.