My DS is only 5 and DD is 3 and they get on well most of the time. Generally he's laidback and she's feisty and it seems to work.
I find it hard to picture their future relationship as I had a younger sister and not much to do with boys.
Will DD moon after his older, cooler friends? Will DS cause havoc with his sister's friends? Do you want to be more strict with your DD than you were with DS? Were older brothers put on earth to annoy their younger sisters or vice versa?
Obviously don't have enough to worry about with two littlies and a third on the way if I'm worrying about them as teenagers