I have two (and we're done) who are 4 years apart. They are pretty close, but also fight a lot. I don't worry about it too much--having read the literature, fighting isn't a big deal as long as it doesn't turn into abuse, and fighting plus playing is more likely to lead to long-term closeness than little fighting but little playing.
One thing I notice is that my youngest does his damnedest to keep up with the elder, which sometimes can be threatening to her. She would like him to be more firmly in the role of little and incompetent than he is. He pushes himself so he can do what she does. I suppose all siblings are like this, but he has the ability to make it work even more. So, when she learned chess through school at 7, he also learned chess...at 3. He even tries to read her chapter books, though he puts them down pretty fast.
