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 -