We've had prompts with Sherlock masturbating to Irene or Mycroft in his mind palace, but I don't think we've had one where he's getting off to both of them at the same time. I'd like to see Sherlock getting off to a fantasy of Irene and Mycroft fucking in his mind palace.

Bonus for Sherlock's fantasy involving Irene dominating Mycroft in some way, and Mycroft commenting on it indignantly.