I have a sister who's 3 years younger than me, and I'm glad I do. However, I'm pretty sure everyone who has siblings has at one point wished they didn't. Unless there's something like a 15+ year age difference, because at that point, the older one is probably grown out of the stupid kid phase. (I was a stupid kid too, just like everyone else. I'm not trying to insult anyone.)
I also think that people who are the only child of the family have a higher risk of being a bit of a jerk, until they grow up at least. That's not a guarantee, of course. It's just that if you have siblings who are about the same age as you, you've likely had to share everything right from the start and your parents probably haven't spoiled you as much as they could have.