"Her sons eat bananas."
Translation:उसके बेटे केले खाते हैं।
उसका is 'his', 'her' or 'its'. उसके is the form of उसका to be used when it is followed by a masculine plural noun.
उनका is 'their' but can also be used to refer to a single person if you are being respectful/formal to/about whoever you are talking about. उनके is the form of उनका to be used when it is followed by a masculine plural noun.
उसके is 'his'/'her' and उनके is 'their'.
But just like 'their' is sometimes used to refer to a single person in English, उनके may be used to refer to a single person as well (when the person is quite older than you or in a position of authority).
So, when you're talking about your sister's kids, it's 'उसके बेटे केले खाते हैं' and when you're talking about your boss's kids, it's 'उनके बेटे केले खाते हैं'.