What is the name of Mary Shelley's stepsister who was at Villa Diodati?

Mary Shelley's stepsister at Villa Diodati was Claire Clairmont. Born Clara Mary Jane Clairmont in 1798, she was the daughter of Mary Jane Vial Clairmont and the stepsister of Mary Shelley after their parents married. Clairmont traveled with Mary and Percy Bysshe Shelley during their European journeys.

In the summer of 1816, Clairmont joined the Shelleys near Lake Geneva, where Lord Byron had rented Villa Diodati. Byron stayed there with his physician, John William Polidori, while the Shelleys rented a nearby house. Bad weather kept the group indoors, encouraging the famous storytelling challenge associated with the origins of Frankenstein and Polidori's The Vampyre.

Clairmont was also Byron's lover and later became the mother of his daughter, Allegra Byron. Her presence is sometimes overlooked because Mary Shelley and Byron receive most of the attention in accounts of Villa Diodati. Jane Williams was a friend of the Shelleys, Harriet Shelley was Percy Shelley's wife, and Fanny Imlay was Mary Shelley's half-sister—not the stepsister identified in this question.