Sigrina Holmes is a sister to Sherlock Holmes named by Philip José Farmer in Tarzan Alive (she is given as the daughter of Siger Holmes and Violet Rutherford. The argument goes that when Sherlock Holmes considers his hypothetical advice to "a sister of mine" in "The Copper Beeches", he is referring a flesh-and-blood sibling.
In Tarzan Alive, Farmer states that he does not believe Holmes ever had a sister, and that the Fu Manchu stories are fictional, but nevertheless includes an entry for Sigrina as Holmes's sister - and as Sir Denis Nayland Smith's mother - in his genealogy (which is, after all, stated in Tarzan Alive to be only analagous to the familial relationships of the originals of various literary characters such as the Great Detective). In later editions of the book, Farmer draws attention to the theory that Fu Manchu was based on the (supposedly) real-life Hanoi Shan, a villain in Warped in the Making: Crimes of Love and Hate and The Thrill of Evil, both by H. Ashton-Wolfe. By the time Doc Savage: His Apocalyptic Life came out, Farmer was ready to concede the existence of Fu Manchu, Nayland Smith, and Sigrina Holmes.
In article "Who's Going to Take Over the World When I'm Gone?: Part II, The Dynasty of Fu Manchu" (published in his book, Myths for the Modern Age), Win Scott Eckert states that Sigrina's husband was John Vansittart Smith from Arthur Conan Doyle's short story "The Ring of Thoth" (which would make Sigrina herself a student of Egyptology). Smith is stated to be related to various other Smiths in literature, particularly in Doyle's own works. Mr. and Mrs. Smith's children are identified as the aforementioned Denis, a brother who apparently died on the Titanic (mentioned in Ten Years Beyond Baker Street by Cay Van Ash, and who has been identified by Wold Newton scholars with the Continental Op and Brad Runyon), and a sister who was the mother of Lancaster Sneed, Nayland Smith's nephew in the Shang Chi comics published by Marvel.
- "The Dynasty of Fu Manchu" by Win Scott Eckert (earlier version of his print article)