I didn't mention it at first because I thought it might be a little complicated to explain on the forum, but I see that I am not the only one to comment about complex family situations, so I will share.
While it is true that I'm my mother's only child, and I didn't grow up with brothers or sisters in the house, I may have half siblings on my father's side.
I was told for some time that I had an older half brother from a relationship my father was in before he married my mother. He was a lot older than me. He was in his late teens before I was born, and I don't remember him well because my mom and dad split up when I was still in daycare and she got custody and moved far away with me in tow.
As I hear it told he was a bit of a problem child, and would get in trouble stealing things, running away from home. I know that he finally settled down and married with children of his own, and became a teacher like my father, and concentrates on helping troubled children and getting them off the streets. I have met him a couple of times. He seems okay. He's friendly.
However, my father recently told me a year or so ago that what I thought I knew was not true. He said he was actually raising someone else's child, and the man I thought was my half-brother was not biologically related to me at all. It wasn't a formal adoption, my dad just brought him into the home and told people that it was his son, and took care of him as if he was his own because this child's mother was not able to. It's like Cosette from Les Miserables or Adele from Jane Eyre.
That's not the only plot twist. According to my father I do actually have an older half sibling, it's just not the person I thought it was. My dad did indeed have an important relationship with someone else before my mother, and says that this woman had a daughter and that he is really the biological father of the girl. I believe the child's mother was married to someone else when she had an affair with my dad, and by the time my dad found out she asked him to not claim paternity and to just keep it a secret. For this reason my dad out of respect never publicly acknowledged this girl he says he is the father of, or even spoke of her to friends or family for many years, but he says he has a lot of regrets that he was not there for this child, and would have made a different choice is he could go back.
He anonymously has helped the person he thinks is his bological daughter financially for many years, and has finally met her. She believes that he is just a generous family friend, and has no idea that she is his real father.
My potential half sister I believe is in her 30s, and was a model living in New York. She has been diagnosed with bipolar disorder and witnessed 9/11 and has a lot of trauma from seeing the towers fall. Because her mental state is fragile it's not been advised by psychiatrists who my father has sought the help of to give her any information that might confuse or upset her, because she is not emotionally ready to hear it. So she still does not know the truth of who her father may be.
I think because he did not have a relationship with this girl, it's part of the reason that he sorta adopted the boy I thought was my brother, out of guilt.