I am an only child. I don't have any brothers or sisters. What about you?
Only child
An only child is a person with no siblings, either biological or adopted. 
Children who have half-siblings or step-siblings, either living at the same house or a different house - especially those who were born considerably later - may have a similar family environment to only children, as may children who have much younger siblings from both of the same parents (generally ten or more years).
Overview.
Throughout history, only children were relatively uncommon. From around the middle of the
saying "The thing is, I don't understand what kids do with themselves any more." He contrasts the overstructured, overscheduled, oversupervised suburban life of the child in the suburban 1950's with reminiscences of his own childhood. He concludes "I guess what I am saying is that people who don't have nightmares don't have dreams. If you will excuse me, I have an appointment with myself to sit on the front steps and watch some grass growing."
"Translations from the English" (1958