Sherlock Holmes, it is well known, plays the violin.
Mycroft Holmes plays the piano.
Gregory Lestrade plays the guitar, a remnant of his punk phase.
John Watson has no discernable musical inclination whatsoever.
Astrid Holmes plays the harp.
It started in primary school: her grandmother died and left her the harp she'd always kept in her parlor but rarely touched. Arthritis, you know. Mum got her lessons, said it was good to know how to play a more obscure instrument. Less competition. "If you ever want to join a professional orchestra or something," she would joke. "Every Tom, Dick, and Jane learns violin or clarinet or piano. Hardly any ever think of learning the harp."
She showed an aptitude for it, played with her school orchestra after a year or two of lessons. She was better than good; she might even have been able to go to a performing arts school to focus on the harp, except for one thing: she hated to practice. It was boring, and dull, and any other word an 11-year-old harpist can whinge at her mum at 5 in the afternoon, on the dot, every day. Of course, if one wants to get in to a good music conservatory program, one practices. A lot. That was the bit Astrid could never quite get right. But then, of course, her mum died, and a lot of things changed. Now she lives with her father, and his boyfriend, and they're happy. Her father plays violin, and his boyfriend likes to listen when it isn't terrible. Not that he's not good. Quite the opposite, in fact. He's brilliant, when he's not torturing the instrument in a fit of pique. The music world has no idea what they lost when he decided not to become a violinist full-time. Astrid listens, too, and learns his style, and sometimes, when the day has been particularly emotional, or her father is playing particularly well, a violin solo will become an improvised duet, the sound of her harp dancing around the voice of the violin, enticing it, drawing out something utterly different than what it would ever produce on its own, and the music is all the more beautiful for that; on those days, John and Mrs. Hudson simply find a comfy chair and a cup of tea and settle in to listen.