Viewers saw Timothy Carlton and Wanda Ventham make guest appearances in the New Year's Day episode, The Empty Hearse, playing Sherlock Holmes's family.
And show boss Moffat - who writes the show with Mark Gatiss - laughed off the casting. In an interview on Radio 2's breakfast show, he said: "Well Sherlock largely runs on nepotism. I'm married to the producer, one of the execs is my mother-in-law, Mark and I are old, old friends, Martin (Freeman, who plays Watson) is working with his partner Amanda Abbington. So generally speaking we just don't like to go outside our tiny circle very much. The absolute fact is that once we came up with the idea, wouldn't it be funny for the first time ever in any Sherlock Holmes film if you could see mummy and daddy Sherlock, the fact is his parents are both actors, really good actors, so it was irresistible."
He confirmed he hoped to make further stories for the popular BBC1 show but said it was a case of fitting in with everyone's diaries.
Moffat was dismissive of critics who carped about apparent holes in plots, pointing out the shows were written for sophisticated audiences who had to fill in the pieces themselves.