Who is Allistair?

In episode 6 "Flight Risk", a person from Sherlock Holmes past makes an appearance. His name is Allistair but at first Joan Watson thinks he is the father of Holmes. After Watson finds out he is not Holmes' father but instead was hired by Holmes to act as his father, she storms out. Later in the episode, Watson begins to question how Allistair found out about a tattoo on Holmes' wrist that covers a scar from when Holmes broke his wrist as child (there is a deeper story to the tattoo but I provided the video above). Watson tracks down Allistair to a book store and questions who he is. Allistair reveals that he is a friend of Holmes and that when Holmes' when on a drug binge, he went to him uttering the name of Irene Adler. So of course after this episode, speculation began on to who he was.

Holmes only trusts certain people and Allistair is, supposedly, someone he does trust. Now there is many things that point to Allistair potentially being Moriarty; for example, like how he works at a book store and this could be alluding to the professor but that seems too easy. Now what if Allistair is the brother of Holmes instead. Allistair appears to know a lot about Holmes' childhood and who else would Holmes trust to act as his father and go to when suffering through a dark time, Mycroft. Another small clue at this is that Allistair even provides a small "brother-type" joke at Watson when asking about "the shagging" between Holmes and Watson. Also, during the scene where Watson and Allistair meet in the bookstore, Allistair seems like he really cares for him. He looks like he is beginning to cry and shown signs of a deep brotherly love for Holmes.