The BBC's miniseries Sherlock was created by Steven Moffat and Mark Gatiss and aired from 2010 to 2017 for four seasons and one special, Victorian-era episode. Sherlock brought the Great Detective, Dr. John Watson, and other characters from Conan Doyle's books to modern-day London, adapting various Holmes cases to a modern setting, technology, and social issues. However, Sherlock's downfall began after season 2's finale "The Reichenbach Fall", and got worse in season 4 with the introduction of a third Holmes sibling named Eurus (Sian Brooke).
The only official sibling Sherlock Holmes has is Mycroft Holmes, so Eurus was an original addition to the BBC's series – however, her introduction and role in the series wasn't done correctly. Eurus was a villain and the most intelligent of the Holmes siblings, making her an evil genius. Because of this, she was sent to Sherrinford, a maximum security psychiatric facility, where she ended up taking control thanks to her dangerous manipulation skills. Eurus ended up designing "games" for Sherlock, Mycroft, and John that put their morality to the test, but the whole scheme was incredibly hard to believe.
Eurus was an unbelievable character due to not being part of the canon, the sudden way she was introduced, and her forced place in the series, and Sherlock & Daughter is at risk of repeating this. Sherlock Holmes having a daughter is hard to believe as Conan Doyle never gave him a love interest (the closest to it was Irene Adler) and he was never interested in any type of romantic relationship. Sherlock & Daughter has to find a believable way of introducing Amelia and making her part of Sherlock's story, unlike everything Sherlock did with Eurus.
Eurus Holmes failed as Sherlock Holmes' sister, but another recent adaptation of the detective successfully introduced a third Holmes sibling: Enola Holmes. The movie, based on Nancy Springer's The Enola Holmes Mysteries series, introduces 16-year-old Enola Holmes (Millie Bobby Brown), the youngest of the Holmes siblings who, after her mother runs away, begins her career as a detective, following the steps of her famous brother. Enola has natural detective skills, though she's not exactly at the level of Sherlock, but she's charming enough to carry her own stories.
Sherlock & Daughter can learn from Enola Holmes in terms of building a likable character who is connected to Sherlock Holmes, yet she's her own person. Eurus was made as a dark mirror to Sherlock, while Enola is similar to her brother but still different enough to successfully be the lead in her own movies and books. Sherlock & Daughter doesn't make it clear if Amelia really is Holmes' daughter or not, but until that is solved, she must have some similar traits while also being different enough to contrast with her "father" and build an interesting dynamic with him.
