Simplify the following text for a Grade 10 native English-speaking person.
Original: The Duke of Holstein-Gottorp , who was himself a member of a cadet branch of the Oldenburgs , married into the Romanov family early in the 18th century ; all Romanov Tsars from the middle of that century to the revolution of 1917 were descended from that marriage .
Simplified: The Duke of Holstein-Gottorp , who was from a younger branch of the Oldenburgs , married into the Romanov family in the mid-eighteenth century .