Her mother was Olive Louise Munday (born Burnard; 1873–1960), Robert Burnard's eldest daughter. Her father was the Principal Medical Officer at the Naval Hospital School in Greenwich. She attended Princess Helena College in Ealing, and then the Central School of Art in London. In 1925 she married Guy Sayer, who was a midshipman in the Royal Navy, and they spent some time in China. Three years later they bought Old Middle Cator, a dilapidated Dartmoor longhouse about two miles west of the village of Widecombe-in-the-Moor in Dartmoor. They had twin sons, Geoffrey and Oliver, born in 1930, and until World War Two the family travelled widely to meet the needs of Guy's navy career. After VE Day, Guy was posted to the Far East and Sylvia settled at Cator and became interested in local politics, at first as a parish councillor for Widecombe, then as a Rural District Councillor and a member of the Dartmoor Sub-Committee of Devon County Council.