Perhaps these are some of the reasons why...
He's so protective; at one point today we heard him yelling, "no! no, baby sister! no!"
He was protecting her from going towards the stairs.
They shared a treat at Caribou Coffee. Owen picked out the cake with the most frosting and then when Grampa was cutting it in "half," Owen knew which "half" to pick: the big one!
I suggested they break it up and Grampa eat fast...then Owen doesn't know how much he ate and how much Grampa ate. :-)