Born on 20 August 1871 at 40 Upper Brook Street in Mayfair, London,[1] Lady Sybil was the daughter of Robert Francis St Clair-Erskine, 4th Earl of Rosslyn and Blanche Adeliza Fitzroy.

Her sisters were Millicent, Duchess of Sutherland and Lady Angela Forbes; Daisy, Countess of Warwick was a half-sister, from their mother’s previous marriage to Hon.

The ball was held in celebration of Queen Victoria’s diamond jubilee, and was one of the most extravagant aristocratic parties ever.

Sybil dressed as Hebe, goddess of youth, carrying a gold cup in her hand, with an enormous stuffed eagle strapped to her shoulders, in imitation of a painting by Joshua Reynolds.

Lady Westmorland died at 58 Queen Anne’s Street in Marylebone, London on 21 July 1910, aged 38, and was buried in the Rosslyn Chapel.