I think that, given the relatively small age difference between Jonah and Owen, we expected the older two kids to have the closest relationship once we learned we were expecting Kate. And they do- make no mistake about it- they really are best friends. But Owen and Kate have really been developing a very special relationship over the past 6 months or so, and I think that this is because their personalities are, shall we say, so very similar.
I love the picture above because, for me, it so clearly shows the different personalities of my three kids: two extroverts monkeying around and one reader just trying to ignore the storm swirling around him.
But for as much as we have lots of boisterous middle brother/younger sister play, we have just as many moments where they are just content to be.
And Now We Are Five 
I love it–that last picture is so, so sweet. And you can absolutely see why Jonah and Lucie get along so well. 🙂