There have been moments when I've hated my bro, but for every such moment, there've been at least a dozen to be thankful for having him around.
Can't imagine being a single child. I think being in pairs is a leg up. You start out knowing that whatever happens out there, you have someone who has your back. It's also a catbird's seat in the sense that you learn to interact and deal with people your age which no adult can possibly provide however great they are with kids, and this kind of social intelligence acquired at home serves well in future which I think a lot of single kids have to learn the hard way.