"Aamir's aunt eats a banana."
Translation:आमिर की मामी केला खाती हैं।
Since मामी is feminine, you need to use 'की'. You use 'के' only for masculine plural words.
In Hindi, words that refer to people elder than you or in a position of authority are conjugated as if they were plural as a show of respect. So, you use the plural form हैं. This is not a grammatical requirement but is usually done in practice.