I do have to say I'm alot closer to my bro, 2 years younger, than I've ever been to my sister, 6 years younger. I think thats more to do with her maturity level though. She's really immature... Maybe one day we'll be closer, but I can't see it happening.
I tried to keep similar gaps between mine, but far enough apart that I wasn't changing 2 lots of nappies, lol.