I was either an only child, the youngest of nine, or an older brother, depending upon who you ask and your point of view.
My dad had eight children with three different women before he met my mother. I didn't know any of them (they're all a lot older than me!) very well, and never lived with them, so I certainly felt like an only child. And then, when I was sixteen, my mother had a daughter with her third husband, (Her second, my dad, died in 1981, three years previous) and I felt more like a babysitter to her than a big brother.
So I'm not sure how you classify me!