JefferyM
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Is this suppose to be a western fantasy? Because, killing a whole noble family for a daughter's murder, would be inviting rebellion, collective responsibility... Even killing a noble daughter without a trial, would be courting rebellion. I know in the east it was somewhat different, but the western names.. Even taking away the families nobility for her crime would court rebellion.
The story king is dead, long live the king. He would not be a prince, in most hereditary monarchies accession is automatic upon death of the monarch. Unless there is some form of electoral process your MC woke up as the emperor not a prince. but then again it is your fantasy novel, but you should come up with some backstory to explain this anomaly..
The wife of an emperor is an empress...
Maybe a grand niece?