@ernesjohnson A slightly different perspective. If my sister and I were not related, we would have nothing in common. We share a trauma-bond, and love each other. Our relationship has had many different forms, one where I was more mother, one where I've been more of a big sister, times when she stepped up to help (saved!) me, and now, she is my best friend. However, I'm also extremely close to my best friend, in that I can talk to either one of them about anything. If I didn't have my sister, I wouldn't be lonely. (As in, if she had never existed. I'd be crushed to lose her now.)
I have 5 other siblings, and apart from happy birthday and merry Christmas, we don't talk much. It's a shame, because my older siblings and I get on great when we are near each other.
I have an only child. My nephew is an only child and is the one who made me realize it's all okay. He said having a sibling would be cool, but he doesn't feel like he's missing anything. He has friends and cousins and now, as an adult, he has an adult life with a partner and pets.