Why is 'ciocią' in instrumental here? I would say it is genitive. Because it is the aunt /of/ him?
It is instrumental because the verb of being "jesteś" ("you are") takes an instrumental.
For example: Jestem mężczyzną. (Nominative= mężczyzna ; Instrumental= mężczyzną)
I typed "Are you his aunty?" and it said that's wrong. Can that be changed?
Apart from the lack of ą at the end, nothing. And that should be accepted with a typo.