If you look at this page, and on a LOT of other pages that talk about words for family, you hear that the on yomi readings for the kanji are usually for “your father” or “your older brother” and that the longer kun yomi words are “someone else’s [whatever].” But I’ve never understood that to be actually true, based on Japanese that I’ve heard spoken myself, and I’m just really confused about when one of these words is used over the other.
Is it levels of politeness? Is it specific circumstances or contexts? Can someone please help me understand?
Basically if you’re telling a third party about your older brother, you’d use あに. If you’re addressing your brother directly or talking about someone else’s brother, you’d use お兄さん.
But in my experience, it’s okay to never use あに. You could always say お兄さん if you want, I’m pretty sure.
Are used to refer to your own family members when speaking to someone outside the family. I’m not really sure why. Maybe it’s to avoid confusing the listener into thinking you are talking about someone else’s siblings since the personal pronoun is usually dropped?
Japanese kids usually don’t call their own family members these names to their faces though. にいさん and ねえさん can be used casually to refer to friends the way “bro” is used in English. Japanese people often refer to their own parents at かあさん and とうさん when speaking directly to them.