I am the only biological product of my parent’s union. My mother had a child with her 1st husband who my father (her 2nd husband) adopted. So he and I grew up together. He is 9 years older than me, though, so we are both like only children.
Then my mother remarried when I was a teenager (3rd husband) to a guy who had an adopted son already. This guy was an adult…~12 years older than me. They had a child 17 years younger than me. Then they adopted a child years later who is 19 years younger than me in age.
Then we found out that my father had a child we never knew about, so I have a sister who is 8 years older than me.
This question is incredibly difficult to answer (logistically).
Shorter answer: I grew up with a brother 9 years older. We are both like only siblings.
Awkward answer: I’m not really sure how many siblings I have. It depends on how I count.