His sister.
I look at the photo that Mycroft hands me and, for the first time, pay no attention as he explains me the various ways the girl sitting in the picture, red hair thrown back, the slight curve of a smile on her lips and the mischievous twinke in her eyes, is his sister.
I don't need to look for signs; this is his sister.
And underneath is written, in Watson's curling handwriting, slightly shaken at the edges, as if while writing it he was under great emotional duress.
Julia Watson.
Born : April 10!847(1)
Died: February 16, 1867
Underneath, Watson had scribbled the following words
May God rest her sweet soul.
(1)I'm using Pompey's fic that Watson was 27 when he came out of the afghan war. That would make his birth date as 1854. Julia is 7 years his senior so she was born in 1847.