E.P.
I had heard 3 years apart is a good age. I tried for that and it worked. I have a daughter and son and they truly love and respect each other - they were born 3 years and a month apart. (She is a second-mother to him... so that I have to watch!!!) I love the 3 years apart just because my oldest has always been an independent soul and really wanted to be part of the action when her brother was born. Plus, I loved those 3 years alone with her.
However, my sisters are 4 years and 6 1/2 years older than I. We too, are great friends.
Good luck to you.