No seriously, he IS the best big brother ever. Which doesn't mean that he doesn't occasionally hit/push/trip/and otherwise irritate his sister. I think that's a perfectly normal (and even necessary) part of being a brother.
The part that blows me away is how she is ALWAYS on his mind. When I pick him up from daycare he immediately runs to her, "Violet! It's my sister!" and procedes to re-introduce his friends/teachers/sometimes parents to his little sister.
And recently he's kind of assumed the role of "Violet thought translator to the parents". For example: If he is asking to color a picture of an animal, he tells us, "I want to color an elephant and Violet wants to color a donkey." (Granted, hers is always the less-cool of the animals... but still.) Or if she is crying, he will tell us that she needs her agua or a snack.
One of my favorite times of day is after dinner when Fernando and I are cleaning up, Daniel and Violet will usually disappear into one of their rooms, playing contentedly. He reads her books. He shows her his toys. He loves to make her laugh. He wrestles and tickles her (often to a "break it up!" place).
My favorite is when he chases her around the house, throws himself on his knees and throws his arms open, "Violet! Nay! Give me hug, Violet!" ("Nay" means "come here" in Ahmaric.) Half the time she obliges, and half the time he gets the shaft... but it is still sweet to see his efforts.
He really, truly loves her.