I love watching my children together. C gets incredibly excited when she sees K. She just lights up, and usually laughs. K also asks C to sing a song a lot, meaning he wants her to play on the piano with him.
Today when I was cooking, K called into me saying "Mummy!", and then ran around the corner to hide. When I asked C where K was, K came running out, and C just smiled. We did that a few times, with C smiling more and more and laughing, until she finally crawled after him. It was very sweet.