My only sibling is my brother, seven years my junior. My dad always says he had "two only children" because with such an age gap, different genders, and drastically different personalities, we kind of were. The only playtime or even lengthy interaction we had growing up was when I was being used as the unwilling babysitter. I had to keep him occupied and out of trouble, so I had no choice but to play with him. It was never much fun. Middle schoolers and kindergarteners rarely have much in common. I much preferred being alone in my room with my books to hanging out with a little kid.
By the time my brother was old enough to talk to on a similar age/life experience level, I was living in another part of the state. We get along alright on the rare occasions we see each other, but it's more like cousins than siblings. We have a little shared history, but no common ground beyond that. Our adult lives, like our childhoods, are very different.
Basically, my early life would have been improved slightly if my brother had never come along. We were just too far apart for him to have a major impact. With my lifelong love of quiet and solitude, I'd have suffered greatly in a large family situation, or even with a close sibling.