As the only child of my father  I have a great deal if responsibility over my  siblings, either biological or adopted
Wow,I understand the demands on you
term "only child" is generally applied only to those individuals who have never had siblings.
OK,what other responsibility do you have?
Some children may have half-siblings or step-siblings, either living at the same house or a different house
Well,that make them all a family
Systems of social stratification
Detailed anthropological and sociological studies have been made about customs of patrilineal inheritance, where only male children can inherit. Some cultures also employ matrilineal succession, where property can only pass along the female line, most commonly going to the sister's sons of the decedent; but also, in some societies, from the mother to her daughters. Some ancient societies and most modern states employ egalitarian inheritance, without discrimination based on gender and/or birth order.
Africa.
Africa Southern Africa.
The system of
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