The "AUNT"/"UNCLE" thing is still used, for adult family friends in South Africa, amongst the Boers. I don't know if it is commonly used by blacks there.
My progeny called GODparents "Aunt/s and Uncle/s", though there was no blood relationship.
Now, if you want to go back a couple of centuries, Southern whites called some black slaves Aunt or Uncle...a la UNCLE REMUS, who was based on a REAL person and his stories.
When I was growing up, we called siblings ‘Brother’ and ‘Sister’ as often as by their first names. We were raised by old people with old habits, but I still call my brother ‘Brother’ sometimes. I have lots of old letters that are addressed ‘Dear Brother’ or ‘Dear Sister’, instead of names.
Old stuff dies hard ;-)
Oh, I see. Your “outdated affectation” is fine but mine is verboten. Got it.