I've often wondered the same thing. My lil sis and I are 4 years apart, we never got a long she was just too young while I was growing up. But now we're fairly close. It's weird sometimes because people think we're twins. Also, now she's dating some people that I went to HS with, it's all to weird to hear about her stories because I pipe in and say, well so and so was such and such back in HS. My older brother is 15 years older than me and we were pretty close growing up. Now he has a family of his own and has moved away so we don't talk much. I think 2-3 years is about right.