I enjoyed being an only child for my first ten years. In my twelth year my mom's back went out and I became a second mother to my little brother. Mom couldn't lift him. When Dad, who traveled, wasn't there, I did the lifting and much of the changing.
I was always the built-in baby sitter. Sometimes I resented what that did to the rest of my childhood and teen years -- even when my brother no longer needed to be lifted or changed and Mom's back healed. It sure was different than being an "only." It was probably good for me, though in many ways. I learned the world wasn't all about me.