I have an older sister, but she's much older than me (read: more than 10 years older). On top of that, she was my parents' favorite, while I was the pariah. We did not get along, so needless to say, I could just as well be an only child. (And after my older sister moved out, I could be considered an only child from a legal point of view.) Since I had no one to play with, spending time at home was boring at best. My parents flat-out refused to get a pet, no matter how much I begged. As a result, during my whole childhood, I had absolutely no one that I could communicate with as an equal. (I had a few friends, but I'm referring family members.) I was a Lilliput in a sea of giants, which made for a very unpleasant childhood, even though I had a lot of toys.
That kind of environment stunted my social development tremendously. Being a Lilliput in a sea of giants, I learned how to follow orders, but not how to communicate with equals (i.e. my peers). So when I found myself among other children, I got bossed around a lot, and that's if anyone let me join their group. I think having had one or more same-age siblings would have taught me important skills in communicating with my peers. Even a dog would have been helpful, because a dog has the intellectual capacity of a six-year-old child, and doesn't give people orders (obviously), making it an "equal" on some level.