Select the most appropriate question tag to fill in the blank. She isn't your…

A question tag mirrors the main clause and flips its polarity: a negative statement takes a positive tag, and a positive statement takes a negative tag. The tag also repeats the auxiliary or be-verb of the statement together with its subject pronoun.

Here the statement 'She isn't your sister' is negative and uses the present be-verb 'is' (contracted in 'isn't'), with the subject 'she'. So the tag must be positive, use 'is', and keep the pronoun 'she' — giving 'is she?'.

Complete sentence: 'She isn't your sister, is she?' The other auxiliaries ('did', 'will', 'can') do not appear in the statement and so cannot form the tag.