I think sometimes closeness between siblings can be based on personality not just age gap, DH is 13 months younger than his sister and had a lot of trouble growing up as she bullied him and completely dominated him. When he started school after a few weeks the school had to insist they be in different areas at breaks because his sister spoke for him and he followed her around and didn't make any friends of his own. They are not particularly close now and never speak on the phone or anything. On the other hand my brother is 4 years younger than me and we have always been close, even more so as adults, we are very good friends and we will call each other regularly. I know I am biased towards longer gaps (maybe not as radical as mine LOL) but its worth thinking about.