As far as I can tell they act like any other brother and sister...they fight, they play, and they, well, everything in-between. They speak for each other and Logan can be very protective of his sister especially with new people. Sophie has started asking for "Brother" when she's upset.
Someone even asked if I felt guilty for Sophie bringing home an older child. My answer to that is a resounding, "No!" I brought Sophie a playmate for Christmas that year. From day 1 they have been playing and running and giggling together. And not once did she hear "Don't touch the baby" or "Not now, I have to feed/burp/change the baby".
They don't always get along but this is how I choose to remember them...helping each other and having fun!