Sherlock and Dexter
I just finished watching the first series (so please no spoilers 😉). Seems this might have been another source of inspiration for Mofftiss. Here is just the important part. Insert Sherlock for Dexter, Eurus for Rudy, and John for Debra:
After finding a clue at Rudy’s house, Dexter arrives at the house of his mother, Laura Moser. Flashbacks reveal that Rudy, whose real name is Brian Moser, is in fact Dexter’s biological brother. He also witnessed the murder of their mother, but did not repress the memory; instead, he grew up in a mental institution being treated for antisocial personality disorder. Dexter meets Brian in their childhood home, where he is keeping Debra. He intends to kill Debra with Dexter as a sort of “family reunion”, but Dexter stops him.
There are a lot of differences between the characters but Dexter - who has repressed his feelings for a long time - realising that he has a “secret sibling” who is a mentally disturbed serial killer really struck a chord with me. Moreover, the secret brother deliberately seeks contact with him, wishing to re-establish their connection.