As with all small children, Kathleen is alternately fascinated and completely oblivious to her new little sister. Kathleen's obliviousness doesn't bother me, and her fascination should, but unfortunately for Sophia, it doesn't either.
To Sophia's occasional discomfort and possible future endangerment, Kathleen's fascination is not one of tenderness and care for her new sister, but one of experiment. We should have been forewarned when I found her one day knocking the head of her doll against her crib rail. Kathleen's first object of interest was Sophia's eyes, then her hands and arms (arms aren't supposed to be wrenched like that). After those grew old, Kathleen moved to experiments of force. She discovered that Sophia's swing won't support two people, that Sophia's carseat doesn't have enough room for the both of them without squishing Sophia's legs, and that her little sister is heavier that at first it would appear. Today Sophia received experimentation at the hands of a pillow, and her skull got tested for driveability by Kathleen's Charger.
A good mother would immediately stop Kathleen's science experiments, but what if Kathleen turns out to be the next Marie Curie? Being a bad mother, I favor watching to see what Kathleen thinks up next. It's much more amusing than just stepping in to stop the fun.