Its a cultural thing, if youre really good friends with someone like they are family but not blood related you call them cousin, or cuz for slang. Or if you have severe brain damage. I say the last one because my uncle has severe brain damage from an accident and completely forgot who my mother (his sister) is. But since shes always there at family events and is close, he calls her cuz. Oddly enough he remembers my siblings and I as his nephews and nieces.