OK, before you say anything after reading this, I know I'm obsessed with main characters having younger sisters! But I promise you, there is a valid reason. This idea just popped into my head after I'd written about Suzie in my Doctor Who story, The Oncoming Storm. I thought; 'What if Sherlock had a younger sister that was just as bonkers and brilliant as him, but also very troubled and, well, Sherlocklike. Wouldn't that be cool?' So here it is! And just to let you know, the words are all mine, but the first case was given to me by the Baker Street Irregulars. Don't worry, I asked nicely first

John had never been one for sentimentality. In fact, that was why, he considered anyway, he had never been able to sustain a relationship for more than 10 months. Yes, that was his record. But compared to Sherlock Holmes, he compared himself to be practically saint-like. Never had he met a man so intelligent and yet so cold and isolated, and yet still manage to save so many lives. He would never have imagined him looking after someone, not even himself. But as always when it came to Sherlock Holmes, John was proved wrong. Again.

He was having a tough day. He had just completed a blog entry for the speckled blonde, and had stayed up all night finishing it. He had popped out to Speedy's to celebrate, and was now shuffling in to 221b, and with a friendly call to Mrs Hudson, walked slowly into his flat. He was greeted with a sight he never thought he would see.

A woman. A young woman. She didn't appear to have noticed John, with her eyes buried into her newspaper, she seemed absorbed and didn't look up when he came in. He could see her face, though. He studied her, intrigued. With, short, brown hair that had a slight wave to it, she seemed fairly short, no more than 5 feet John guessed, and he presumed her age at about 25. There was something familiar about her features, but what that was he wasn't quite sure. When she spoke he jumped, absorbed in her delicate cheekbones. Her tones were sultry and fairly deep, but not strangely so. More that she was well spoken and classy. Very classy. What she said wasn't particularly classy, though, and this juxtaposition took John by surprise.

'Milk, no sugar please, I'm watching my weight,' she purred, without glancing up from her newspaper. John shuffled awkwardly from foot to foot, unsure of what to say. This sudden exclamation also made him realise that she must have heard him come in, and at this suggestion his cheeks flushed and he coughed, embarrassed. She looked up. 'Oh, sorry. You're not Mrs Hudson?' She said almost quizzically.

'Uh, no, not since last time I checked,' John muttered awkwardly, before he realise that her utterance was a declarative, not a question.

'I just came up. I thought this was Sherlock's flat.'

'Oh, it is, I'm just his flatmate, Dr John Watson. Hi, nice to meet you, and you are?...' He held out his hand. She stared at it for a few moments, before he let it drop limply to his side. She appeared to relax a little though, and folded up her newspaper and stood up, and proceeded to circle John, sometimes coming near, sometimes keeping her distance. It was obvious she was trying to make him at ease, but unfortunately this did not have the desired effect with poor John. Again, this lack of social understanding was somewhat familiar, but he pushed the thought out of his head. There were more dire and important things to focus on now than to take a stroll down memory lane. He hunched up his shoulders in panic, not sure how to react. She spoke again, this time louder and more confident.

'I'd ask you if Sherlock's told you about me, but I should think that the answer's probably no. He does like to protect me, you see. You wouldn't think it would you? Is that why you look so surprised?'

Oops, he looked surprised, did he? Quick hide the emotion, the similarities between this woman and… someone were overwhelming. Fortunately, the mysterious young woman put him out of his misery.

'You don't recognise me? Well then, I shall introduce myself to you.' She smiled. 'My name is Cassia Holmes. I'm Sherlock's baby sister.'

And that was when John realised who she was similar to.

20 minutes later

Just after John realised why Cassia was so familiar, her older brother walked in. He was furious of course, but after Cassia explained her predicament; her brain was practically rotting she had so little to do, and needed a case to help her brother with; he relaxed and introduced her properly to John. He learned that his guess of her age was bang on, which meant there was quite an age gap between her and Sherlock, and an even larger one with Mycroft. They all chatted happily for a while, mainly consisting of Sherlock and Cassia arguing about tobacco ash. John was quite happy to watch them, he realised just how similar they were and figured they got on very well, better than either of them got on with Mycroft he imagined. However, their playful talk was interrupted when a young woman practically fell through the door, causing them all to look up in shock. She seemed to be in deep shock; she was shaking and breathing quickly, suggesting she had just run a long way. Cassia was the first to react; she jumped up from her seat and gently guided the shaken woman, who looked about a year older than the Holmes sister, to a chair, before John could even stand up. This struck him as being different to Sherlock, indeed he was still in his chair, looking more alert than John imagined he himself looked, but in no position, it seemed, to assist the woman. Maybe Cassia was just that bit more compassionate than her ice-hearted brother.

Sherlock spoke briskly to the woman, not even bothering to comfort her. Cassia kept her arm round her; it seemed to hold her together. 'Who are you, where did you come from, and why are you here? Start from the beginning; don't be boring.' The woman looked stunned, and Cassia gave her shoulder a reassuring squeeze, and, unsmiling, nodded at her to begin.

'H-hello, my name's Sophie, I just ran away because-because…' she broke down into floods of tears, and Cassia shushed her and gestured for her to continue. Sophie composed herself and began again. 'I've just been kidnapped. I mean I have no idea why anyone would want to kidnap me, I'm just a very uninteresting chambermaid in a hotel, I'm nothing special, I've done nothing with my life, I have nothing of any use or anything of great value, what could they possibly want from me?'

Sherlock nodded, absolutely no compassion showing on his cold, sharp features. 'I can instantly deduce that you yourself have absolutely no value whatsoever, I mean you're right, look at you, no use whatsoever, but you're wrong about one thing, Sophie. You must have something that the kidnappers want. It is very rare that they would take you just on a whim. There has got to be reason, and it must be something in your possession. Is there anything that you might have that they could possibly want? Anything that would persuade me right now that you are, in fact, useful? Because now all you're convincing me of is that my sister here puts her confidence in the wrong people. Why are you comforting her, Cassia, when she's just as useful as you are? Which isn't much, by the way.'

John was flabbergasted by this sudden outburst, Sophie looked hurt and shocked, and Cassia just had this awful sort of blank look on her face, as if she didn't want to acknowledge what he was saying. Either that or she was so used to it by now that she had learned to shut it out. Surprisingly, it was Sophie who was the first to speak after this uneasy silence.

'Honestly, I can't think of anything I might have that they want. But… Sorry, what was your name again?'

Cassia spoke up; her face a lot brighter than it had been only moments before. 'I'm Cassia Holmes, this is my brother Sherlock and this is Dr John Watson, Sherlock's flatmate.' The two men nodded at Sophie respectively, and she settled back into the chair, looking considerably more relaxed than when she had first arrived not two minutes ago. Cassia spoke gently, egging her on to continue. She did so graciously. John couldn't help wondering that Sophie was actually older than Cassia. Physically, she certainly looked it, but knowing Sherlock he supposed that Sophie's intellect was no match for Cassia's, if her brother was anything to go by. Also, at that precise moment Cassia certainly seemed older, in her manner, definitely. Sophie was obviously much shaken, and Cassia tended to her in ways that neither John nor Sherlock, would have done. Her instincts seemed almost maternal. But then, John thought, maybe it was simply because she was female. Or it could have been looking after her two older brothers all her life and he shivered at the prospect. He would never understand the mind of a woman. Or Sherlock's for that matter. Sophie's sweet voice shook him from his daydream, and he then realised just how attractive she was.

'I moved here from Germany a couple of years ago. I brought barely anything with me, and all my possessions have more or less been bought here in London where I have lived ever since. So you see Mr Holmes, I cannot possibly have anything of great value.'

Sherlock glanced at John to seek his perception on the whole thing. He was looking rather odd, even more than usual, in Sherlock's mind. He was gazing quite dreamily at Sophie, and with a groan Sherlock resigned himself to the fact that John obviously liked Sophie. Normally Sherlock knew that it would be over in two months, but now he wasn't so sure. He even detected a hint of jealousy in the corner of his overused, emotionally empty brain. He didn't like it one bit.

When they all separated off to do tedious jobs (Cassia made tea, John went out to the shops to get milk and Sherlock went into his room to think) they made the mistake of not keeping an eye on Sophie. Sherlock walked back into the room a couple of minutes later and with a jolt realised that she was gone.

'John, Cassia!' he hollered, and they both came running quickly, finished with their small tasks. In all his jealousy, Sherlock claimed that she must have wandered off, but John was insistent that the kidnappers had finally got her. Without another word, John grabbed his coat and furiously stormed out of 221b to look for her, leaving Sherlock and Cassia standing there looking shocked.

2 weeks later

John had been looking for Sophie for a fortnight. He had been to the police, even had Lestrade on Sophie's case. But every night he would walk in with the same resigned expression, and shuffled off to bed. Cassia had taken to sleeping on the sofa; it turned out that she was somewhat of an insomniac and would often wake restlessly in the night and wander round the flat looking for things to do, and did not wish to wake her two flatmates. It seemed obvious to John that she was going to be staying for a while, and if he was perfectly honest, he didn't mind that one bit. He rather liked her wit and wisdom, and her ability to out-talk Sherlock and turn any situation into calm and organised one. Everyone just seemed to relax around her, and John admired that.