She was the first wife of Roland Cubitt, 3rd Baron Ashcombe, and, through her only daughter Rosalind, was the maternal grandmother of Queen Camilla.

Sonia's mother Alice was a British society hostess and a long-time royal mistress of King Edward VII.

[1] Sonia's only sibling was the English writer and socialite Violet Trefusis (née Keppel).

Roland Calvert Cubitt, at the end of the First World War, shortly after the signing of the armistice in November 1918.

He was the fourth of the six sons of Henry Cubitt, 2nd Baron Ashcombe; all three of his older brothers were killed in action during the First World War.

Roland was heir to Ashcombe Barony and to a fortune established by his great-grandfather, Victorian builder Thomas Cubitt (1788–1855).