My brother and Ihave a 2 year gap, and untill he became an adult (20+) we have always fought like hell. He was a difficult baby and was terrible twos, so was very jelous of the attention I got. thats why we went for a smaller age gap, so noah will be too young to understand and get jelous.
But it varies, I had a couple of brother/sister friends who were all very close, so Iguess you never know and it depends on the child.