It has it's positive and negative points.
My sister and I are oldest and youngest (I'm the youngest) so growing up she always mothered me and we got on great. While my brother and I are closer in age and we played a lot together as kids, I think the boy/girl thing caused a few clashes and we were literally killing each other at times.
As I've got older it's reversed. We've all been through personal hardships, as well as family hardships which brings you closer together, and from the age of about 13 my brother has been my best friend. It killed me when he moved to France for the year, and now that I'm going away to uni and he's going back to France, while we're both excited, he's the person I'll miss the most.
My sister and I went to the same primary school, same grammar school and went to dance classes together so did everything together, and after a while we got on each others nerves
we're alike in some bad ways too- both stubborn, sharp tongues etc. When she moved away we definitely got closer though so we get on pretty well now