December 7: "Watson's sister" (from Michael JG Meathook)
This prompt surprised me! I've never had a headcanon including a Watson sister. I would have liked to do more, but my inspiration and motivation is lower today!
Holmes was not one to involve himself with weddings, but when it came to Watson, the detective often found himself making exceptions. The dear fellow had all but begged him to be his best man.
As far as Holmes knew, his dearest friend was without family. His father and elder brother Holmes knew had passed, and Holmes could not recall his friend ever leaving to visit family or vice versa.
It was with great surprise, then, that Holmes was greeted at the wedding rehearsal dinner by a woman whose face was unequivocally like Watson's. The same cheekbones, the same brows, and even the same hazel eyes. For a moment, Holmes was too startled to deduce anything at all about her beyond the obvious.
"You are Watson's sister."
"And you are Sherlock Holmes." She smiled. "Very pleased to finally make your acquaintance."