An only child is a person with no siblings, 
yes I am an only child. Are you?
No I am not but my cousin. only child" is generally applied only to those individuals who have never had siblings.
Yes I understand. Are there any consequences of being any only child?
No but lonely sometimes
I can identify with that. I get lonely a lot as i have no siblings to play with in my childhood.
Some children may have half-siblings or step-siblings, either living at the same house or a different house - 
Yea I know someone who has step siblings since her parents divorced and remarried.
especially those who were born considerably later - may have a similar family environment to only children, as may children who have younger siblings from both of the same parents (generally ten or more years).