Following the Trail of Irene Adler's Sister

Sherlock Holmes knows that Irene Adler and Godfrey Norton didn't escape the island alone. He realizes that behind Irene Adler's blackmail and extortion there is a mastermind whose handwriting is remarkably similar to hers. However, this mysterious woman, whom no police officer has been able to identify because she possesses multiple identities and disguises, in many ways, is a very dangerous criminal, even linked to the murders of Jack the Ripper, and involved in the separatist movements in Ireland and Scotland.
She also intends to end the power of the British Empire by instigating an uprising in the colonies. But, who is this woman? She isn't British, but she lives in hiding, in British territory, and her name is a complete enigma. Although Sherlock Holmes manages to identify her, she kidnaps Dr. Watson, and if Holmes's rescue plan fails, they will both die... So, who is Irene Adler's fearsome sister?