'Is this your sister?' -----> 'Yes, it is.' 'Is this your sister?' -----> 'No, it isn't. 'Is that your sister?' -----> 'Yes, it is.' 'Is that your sister?' -----> 'No, it isn't.' 'Is she your sister?' -----> 'Yes, she is.' 'Is she your sister?' -----> 'No, she isn't.'

"Yes, she is" or "Yes, she/that/this is my sister" negative reply; "No, she/that/this is not my sister" "No, she isn't" You wouldn't say 'yes IT is' because then you're not talking about a person but an object. :)