a sister or a brother
In terms of Mary Shelley's biography, this may be a revealing, if somewhat odd, statement. Mary Shelley was the sole child of William Godwin and Mary Wollstonecraft, conceived before they were officially married and then left motherless by Wollstonecraft's death. She had three step-siblings: Fanny Imlay, the daughter of Mary Wollstonecraft and the American Gilbert Imlay, who committed suicide in 1817 while Mary was still engaged in the writing of Frankenstein; and Charles and Claire Clairmont, children of the first marriage of Godwin's second wife Mary Jane Clairmont. Because of their gender and nearness of age, Mary and Claire were thrown much together and experienced considerable sibling friction. Claire's unacknowledged pregnancy by Lord Byron was the driving force behind the 1816 journey to Geneva, and when her condition became known it fell largely to Mary to see her through the pregnancy without scandal. From this point on, for the next five years, Claire lived with the Shelleys both in England and Italy. Whatever words Victor speaks here, it is clear that Mary's closeness to her step-sister generally increased the suspiciousness with which she regarded Claire's motives and actions.