My brother and I are about 15 months apart and get on brilliantly. Whilst my cousins (2 girls) have 3 years between them and have never got on as well and are really competitive.
I know there might be no link whatsoever in this case and there'll be masses of exceptions but if we generalise do you think it's easier in some senses having children close in age from the point of view of them getting on well?