The following sentence is taken from a book:

Louise is stealing from my office... or is she?

My question is this:

Why is it "or is she?" and not "or isn't she?"

Thank you


Those combinations are always framed as a statement followed by the affirmative question, not a negative. The question throws doubt on the truth of the claim just made. Such a question is equivalent to "Or maybe not".

This plan is going to work — or is it?
Giacco will surely win the game — or will he?
Our kids are always well behaved — or are they?

The negative question would occur after an affirmative question.

Does she want to go? — or doesn't she?
Was the man innocent of the crime? — or wasn't he?
Has the boss decided to approve the project? — or hasn't he?