I think it's one of those things.
My 4 yr old and my 5 yr old are very close in age and they are best friends, everyone comments on their very close entwined relationship and outsiders usually presume them to be twins. I have concerns actually for next year as one will start school and one won't and they have never been separated. I have older children and as much as they love their little sisters, they certainly don't have this close relationship with either of them. Maybe it will change as they get older, who knows?