Sherlock Season 4: 20 Easter Eggs & References In 'The Final Problem'
9. Moriarty's Brother
Moriarty returns in this episode in the form of a series of amusing/threatening videos that he apparently recorded before his death. Eurus explains that Jim enjoyed doing them as he was jealous of his brother, who was a stationmaster.
This is a reference to The Valley of Fear, in which it is said that Professor Moriarty has a brother who works as a stationmaster.
In Doyle's The Final Problem, Moriarty is also mentioned as having another brother called Colonel James Moriarty - who weirdly posseses the same first name as his criminal brother.