When John Watson walked into 221B Baker Street, he knew something was wrong. It was quiet, but it shouldn't have been. When he had left the flat Sherlock had been bust tinkering away with his science equipment and would normally be at it for hours, but John had only been away for a hour. So when he walked into the living room, John was surprised to see Sherlock sat in his chair, with Mycroft Holmes and a young girl sat with him.
"Ah John, nice to see you've come back. Before you ask, this is my sister Sophie. Yes, I do have a sister and no, I haven't mentioned her before." Sherlock quickly shot at his flat mate. John just looked at Mycroft with confusion written in his features.
"What Sherlock has just told you is the truth John. Our sister Sophie will be staying with the both of you for a while." Mycroft said glancing at both Sherlock and Sophie. John, finally able to speak again, just turned to look at Sophie and asked, "how old are you Sophie?"
"She's fifteen", both Sherlock and Mycroft responded together, Sophie just looked annoyed.
"I can speak for myself you know", an irritated Sophie responded. John just laughed.
"Ok then, if you don't mind me asking, why are you staying here? Not that I mind or anything, it's just that I've never heard of you, and, ill stop babbling now", John quietly trailed off, looking a tad flushed in the face. Mycroft just looked to his brother, and then at his sister and said, "Sophie will be staying here, as she is in danger. Sadly, it's not the normal danger that me or Sherlock can easily fix because Sophie is being hunted. Hunted by a Sebastian Moran, one of Moriartys spies."