I am an only child twice over. I did not inherit siblings when my mother remarried My biological father died of MRSA after years of being bedridden with a stroke brought on by agent orange, which also presumably caused my spina bifida. So there was a while when my mother didn't get much help in parenting, and this affected me greatly (though I'm so glad she didn't rush into marriage just to give me a father figure again. She waited for the right guy, and has been married to him for over 20 years).
My mother and I have a difficult, draining relationship. We just have varying styles of communication. That's all. Good intentions all around, but my tolerance for differing styles of conversation is low.
My mother has a great relationship with her family, particularly with her only sibling, her brother, my uncle. I envy that. It makes me wish I had a sibling growing up, but my mother keeps saying the first and only time was the charm.
I had trouble making friends growing up. I was always the only disabled person at school, and in high school, I moved around, attending 4 high schools in 3 states. And to this day, I still have social trouble. I wish I had siblings (two max, I think). They could be neutral bodies during arguments with my parents.