@Bod. I agree about personality. My big sister is only a couple of years older than me. She could have been great for me, but she was a bitch to me, and one of lifes failures. I got bullied at school, and she would be friendly with my bullies, and continue picking on me at home. Now she wonders why I let her live in poverty, and wont do a thing to stop it.
And I think circumstances of the child have a big impact on how much theu need siblings. If the child is lonely, and doesnt have many friends, then a siblong could makr a huge difference. I was lucky and met mu best friend before I started school. Since then, Ive been able to make her friends into my friends.