I agree that it is about personality too, but age gap can be a factor. There is 6.5 years between my youngest sister and I (there is one in the middle, 3 years younger than me). We never had anything in common growing up as I was in grade 6 when she was in prep. We weren't at high school at the same time at all. We were at such different stages. We get along well now (although she is overseas so we rarely see each other) but as kids we loved each other, but didn't really play together or anything. I guess that's something to consider, that it's not just the gap between any two siblings, but also the gap between oldest and youngest.