California Uncle

A California Uncle is a man who has previously had intimate relations with a woman; who has had a child by another man. A man is not a California Uncle until relations with the mother have concluded amicably

“Bro I’ll up later, I have to get a graduation present for my niece”
“Your sister is 20, how do you got a niece?”
“Nah not like a blood niece, I’m a California Uncle”