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Can you? Yes. There is nothing wrong with it, either legally or ethically. What you're doing would come under "artistic license". In fact, Alexandre Dumas' "The Man in the Iron Mask", for instance,...
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#4: Attribution notice removed
Source: https://writers.stackexchange.com/a/34669 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#3: Attribution notice added
Source: https://writers.stackexchange.com/a/34669 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
_Can_ you? Yes. There is nothing wrong with it, either legally or ethically. What you're doing would come under "artistic license". In fact, Alexandre Dumas' "The Man in the Iron Mask", for instance, relies on similar artistic license: as far as we know, Louis XIV had no twin brother. So you'd be in good company. _Should_ you? That's rather opinion-based. Here's something for you to consider, however: unlike Dumas' original readers, your readers would have access to Wikipedia. They'd immediately know there was no such sister. For some, that might significantly impede their suspension of disbelief. Then again, the fact that Grand Duchess Anastasia Nikolaevna had been very definitely murdered together with the rest of her family, did not prevent Disney (or was it some other studio?) making an animated movie about how she actually survived.