Final Sherlock theory:
Moriarty is totally Sherlock’s half-brother, and Mummy Holmes is behind Moriarty’s face on all the televisions. Mycroft isn’t prone to bursts of brotherly compassion… “you know what happened to the other one.” Another brother to whom something bad happened that Mycroft didn’t stop? Hmm. Mummy Holmes is a genius- a mathematician who “gave it all up for children,” who’s published some very impressive books on mathematics. Know who else was a mathematician who’d published impressive books? Moriarty in the ACD stories. If Mummy ever found out who shot Sherlock (“put a bullet in my boy”) she was going to come over “absolutely monstrous.” Sherlock is about to be sent on a mission to Eastern Europe that would result in his death within six months, and the only thing that could stop it would be a threat to England that required his presence. His genius mother might find a video she had of her other son in which he asks if she missed him and use it to frighten the country enough to keep her baby from being sent on a death mission.
The only question is if she was involved in any other aspects of her other son’s life- say, his crimes? Not his vendetta against her legitimate children, I’m sure, but is Mummy Holmes actually a criminal mastermind? Or just a doting mother who created a national emergency to protect her son?