Simplify the following text for a Grade 14 native English-speaking person.
Original: Oliver Cromwell , the revolutionary leader who overthrew the British monarchy and led a short-lived republican government in the 17th century , was a descendant of Thomas Cromwell 's sister , Catherine Cromwell ( born circa 1482 ) .
Simplified: Oliver Cromwell , the Parliamentarian leader who overthrew the monarchy during the English Civil War , was a descendant of Thomas Cromwell 's sister , Katherine Cromwell ( born circa 1482 ) .