Which daughter of Lord Grantham marries Tom Branson?

Lord Grantham's daughter who marries Tom Branson is Lady Sybil.

Sybil is the youngest daughter of Robert Crawley, the Earl of Grantham, and Cora Crawley. Unlike her more conventionally aristocratic sisters, Mary and Edith, she is politically progressive and increasingly uncomfortable with the restrictions of upper-class life.

Tom Branson begins as Downton's chauffeur and is an Irish nationalist with socialist views. His political outlook and working-class background make the relationship controversial, especially within a family that expects Sybil to marry suitably. During the First World War, Sybil works as a nurse, while her connection with Tom grows stronger.

The couple initially plan to elope, but eventually receive Robert's reluctant blessing and marry in Dublin. They later return to Downton with their daughter, Sybbie. Viewers sometimes confuse Sybil with Sybbie: Sybil is Tom's wife and Robert's daughter; Sybbie is Tom and Sybil's child. Sybil dies after childbirth in the third series, but her marriage remains one of the show's defining romances.