Sherlock Season 4: 20 Easter Eggs & References In 'The Final Problem'
17. Sherlock's Hidden Trauma
It turns out that Sherlock did not remember his sister because of the trauma surrounding the death of Redbeard (more on him later). This is said to have resulted in him becoming the removed, calculating machine he is today.
This recalls Nicholas Meyer's novel The Seven Per-Cent Solution (and also the movie adaptation). In that, Holmes visits Dr Freud and uncovers the fact that he invented Professor Moriarty the master criminal. The man he sees as his nemesis was actually his mother's adulterous lover who contributed to her death when Sherlock was a child.
The idea of Sherlock losing a loved one as a child is also lifted from Young Sherlock Holmes. In that movie, Sherlock loses the love of his young life, Elizabeth Hardy, which causes him to forego romantic attachment for the rest of his life.