Fanny Wollstonecraft Godwin was Mary Shelley's half sister, from the same brilliant mother - Mary Wollstonecraft, author of 'A vindication of the Rights of Women'
However whereas Mary's father was William Godwin, the thinker and social reformer, Fanny's natural father was Gilbert Imlay, whose mother was of African descent. In a description of Fanny in the paper after her death, they mention specifically her 'dark complexion' and long brown hair.
Could Mary Shelley have had a sister with brown skin? it would explain Fanny's spiral into depression and her struggle to assert her right to a decent life in the Regency period, her overwhelming sense that she had no future, and her tortuous relationship with Percy Shelley.
A short novel about her life, her loves and dreams, spoken mostly by Fanny herself, as a ghost memoir.