I am an only child.  I have no brothers or sisters.
Are you upset about that?
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
part:
First, I want you to know that Brotherhood is a reality with me. You are all my brothers and sisters, children of the same great Father that I am, and entitled to all the justice and fair treatment that I want for myself. And so long as we run this shop [which to Nash meant three or four months longer] God being my helper, I am going to treat you as my brothers and sisters, and the Golden Rule is going to be our only governing law.