In 1560 Lady Catherine secretly married Edward Seymour, 2nd Earl of Hertford, the son of Edward Seymour, 1st Duke of Somerset, who was a brother of Henry VIII's third wife Jane Seymour. As if her cousin's marrying anyone without her permission was not enough to anger Queen Elizabeth, Catherine's marrying the nephew of the woman Elizabeth's father had executed her mother to marry pushed Elizabeth into a full-blown fury. She imprisoned, for a time, everyone who had anything to do with it, including Bess of Hardwick, who was the one Lady Catherine confessed to when she became pregnant and knew the secret was about to come out.
The marriage was annulled in 1562 but resulted in two children: Edward Seymour (1561-1612), 3rd Earl of Hertford, and Thomas Seymour (born 1563). It was this Edward Seymour whose son William Seymour, 2nd Duke of Somerset, got into trouble by his secret marriage to Arbella Stuart, another cousin with an attenuated claim to the throne of England.