A/N: well I've got some great feedback on the first chapter so looks like this story is full steam ahead! And thank you to Larry Holmes who made me realise that 'chalk and cheese' is a british saying - I thought everyone used it! Just means that they're the complete opposite, in case you didn't get it :) and maybe I can work 'thick as two short planks' into it at some point too..

"So," Castle said, "you gonna tell me why you failed to mention for the past four years that you have a sister?"

"Four years?!" Charlie sputtered before Kate could reply to Castle. "You two have been together for four years..and we didn't know about each other?"

"We aren't together," Kate replied quickly and she could have sworn she heard Castle mutter "yet" just loud enough for her to hear. She was pretty sure her walls were a messy heap on the ground right now, it wasn't going to take much more before she let him in for good. "We're partners. At work. Castle shadows me on cases."

"Right," Charlie said skeptically, "and the two of you aren't sleeping together?"

"Charlotte Harper Beckett will you grow up?" Kate scolded while Castle just laughed.

Charlie grinned at her sister and said "oh come on, you're not the same person I left behind five years ago, there had to be something to make you like this."

"Like what?" Castle and Kate asked simultaneously and Charlie just smiled at them both. "Happy..full of life..joking about..you haven't been like this since.." She trailed off, not wanting to bring up their mothers murder, especially when she didn't know what Castle knew.

An awkward silence fell and Castle, ever the class clown broke it. "Well, I've been trying to get in her pants for four years now but she's having none of it. Perhaps you could talk some sense into her?"

"Castle," Beckett hissed, "do you really have to tell my sister that?"

"Four years, huh," Charlie interrupted. "You've been waiting four years and you haven't slept with her once?" Castle shook his head and Kate groaned in embarrassment. Were they really having this conversation? "So why are you still around? Surely you figured out a long time ago that she isn't going to be easy, so why are you still here?" Yes, yes they were.

"Castle, you don't have to answer-"

"She's worth it." Was all Castle said, interrupting Kate and Charlie's eyes widened. She was about to press for more when her sister stopped her.

"Okay, so how long are you in town?"

Charlie shrugged, "a month..a year..forever?"

"Really? You're back for good?"

"Yeah, I guess I am," Charlie smiled just as their food arrived. "God I've missed New York burgers," she said as she took a huge bite.

"So, are you going to tell me now why I didn't know you had a sister?" Castle asked. He couldn't believe this was happening, he thought he knew Kate Beckett inside out - apparently not.

Kate was saved from answering by Charlie. "Kate and I have hardly spoken in five years," she said, "I left for England the second I got my degree and I've been teaching there and in developing countries ever since. When our mom died," she heard Kate take in a deep breath, but the look on Castle's face suggested he already knew quite a lot, "we took it different ways."

"Let me guess - while Beckett was using every spare hour as a rookie to break into the files, you put it to one side and accepted it?"

Charlie looked from Castle to her sister. "How..how does he do that?"

Kate shrugged, she was kinda used to him doing things like that now, it was just something Castle did. "First time I met him he figured out someone close to me had died, I was never planning on being a cop and that we didn't have justice for mom."

Charlie looked at Castle, who also shrugged. "Writers intuition?" He suggested and something in Charlie's brain clicked. Writer? Wait..

"Oh my god, you're Richard Castle?" He nodded, feeling a little confused. He thought they'd done the introductions back at the precinct. "As in, the Richard Castle?"

"Er, I'm sure there's other people with my name but if you mean the author, then yes, that is me."

Kate had a feeling she knew where Charlie was going with this and she wasn't sure she was going to like it. "But you're Kate's favourite author!" Charlie blurted out. Oh no, not going to like this at all. "I remember now, she had all of your books..wouldn't put them down, she was obsessed!" Still am, Kate added silently. "Wait, if you're an author, what are you doing following my sister around at work?"

"Castle used his friendship with the mayor to talk his way into my team and we haven't been able to get rid of him ever since. Apparently the couple of bestsellers he's produced since have a major part in that too," Kate said.

"Wait, Nikki Heat?" Charlie had read them, another American teacher had brought them over a few months ago and said they were great, so Charlie had given them a try. "You're Nikki Heat?" She asked, eyes narrowing at her sister.

"Apparently so," Kate muttered.

They finished their lunch, conversation flowing about Charlie's time abroad and what Kate had been up to in the past five years.

"Er, Charlie..where are you staying?" Kate asked, however she had a feeling she already knew the answer.

Charlie smiled sheepishly. "Okay, so you know how I'm your favourite sister?" She began.

"You're my only sister."

"Yes, well..I'm staying at yours."

"Charlieeeee," Beckett groaned, she should have known this would happen.

"Oh come on, Kate, it'll be fun! It'll only be for a few weeks until I can find myself a place and anyway, we have an awful lot of catching up to do," she said, her gaze flittering from her sister to Castle and Kate groaned again. As soon as they were alone tonight, Charlie was going to beg her for information and she'd been the only person Kate could never lie to. This was going to be so much fun..

They paid the bill and headed back to the precinct, just so Beckett could tell the others she was done for the week. She had been planning to finish early today anyway to meet Charlie, so it wouldn't be too much of a problem. The three of them exited the elevator and both Ryan and Esposito did a double take when they saw two Beckett's.

"Er, something you wanna tell us, Beckett?" Ryan asked.

"Guys, this is my twin sister, Charlotte. Charlotte, this is Detective Esposito and Detective Ryan," she sighed, doing the introductions again.

"Charlie," she announced, shooting an evil glare at her sister and shaking the hands of both detectives.

"So uh, wanna tell us why we've never met your sister before?" Espo asked.

"Long story," Beckett muttered. "Anyway, we just came up to tell you we're off, I already cleared it with Gates for me to leave early today, see you Monday?" Castle and Charlie began heading back to the elevator as Kate said goodbye and she didn't miss where Espo's gaze currently was. "Eyes off my sisters ass, Javi," she grumbled. God, her sister had been back all of two hours and she was worn out.

"C'mon Beckett, I'm just appreciating art," Javi replied with a grin.

"Hmm, and what would Lanie think?"

"We aren't..I mean..we're not.." He stuttered and Kate laughed.

"Whatever, see you Monday," she called to them both before following her partner and her sister.

An hour later and Kate had dropped Castle back at the loft with the promise to call him tonight so they could talk more. She felt bad for not mentioning Charlie before, but like Charlie had said, they'd hardly spoken in five years and they needed to catch up before she spoke to Castle again. They'd got back to her apartment to find Charlie's stuff already here, apparently she knew someone who owed her a favour, and he'd dropped her luggage off.

"So," Charlie began, sinking onto the sofa. "You gonna tell me what the deal is with you and Castle?"

"There is no deal, we're partners..friends," Kate said, before looking to her sister to see her looking at her expectantly. "What?" She asked.

"Katie, this is me you're talking to, remember?" Kate sighed at the use of her nickname and sat down next to Charlie on the couch. "I'm your other half, literally. You've never been able to lie to me and I'd never want you to. I know I probably went a bit far earlier with the whole sleeping together thing, but I honestly thought you were together. Why didn't you tell me he was following you?"

"Oh that would have been a nice email, 'hey sis, I hope you're okay today with it being moms birthday. Oh, and by the way my favourite author has been following me around at work as research for his novels. Speak soon!' No, I don't think so," Kate said and Charlie laughed.

"Okay, you've got a point. But don't think I haven't noticed you didn't answer my first question," she reminded her and Kate groaned.

"Well I wasn't lying, we are partners, and we're friends..but.."

"But?" Charlie pressed and Kate knew she wasn't getting out of it.

"He's so much more than that. He's my best friend and I can tell him anything and I know he'll be there for me, no questions asked. He's proved on more than one occasion that he'd do anything for me - he tried to take a bullet for me, Char."

"He..what?" Kate had told her in an email about being shot so that Charlie didn't find out on Facebook or something, but she hadn't said anything about Castle wrestling her to the ground. And what he'd told her.

"Yeah, when I was shot he saw it just a second before. He pushed me to the ground, on top of me but it was too late, I'd already been hit."

Charlie could tell she was still holding something back. "And what else aren't you telling me?"

Kate damned her sister for knowing her so well. "He uh, he told me he loved me," she admitted.

"So the guy has been there for four years, not taking advantage once, tried to take a bullet for you and told you he loved you but you're still not together?" Kate shook her head. "Why, do you not love him back?"

"Oh god, I love him so much, Charlie. So much that it scares me, I'm scared I'm gonna screw it all up and he'll hate me," she admitted.

"Kate, I saw the way he looked at you, he could never hate you."

"You think?"

"Kate, I know," she said taking her sisters hand, "sweetie, if I had a guy look at me the way he looks at you I'd throw myself at him. That guy is madly in love with you and you need to tell him you feel the same."

"You sound like Lanie," Kate laughed.

"Who's Lanie?"

"My other best friend. Oh, you'll love her, she's the ME we work with on most cases. She's brilliant and she reminds me so much of you, won't take no for an answer, keeps pushing until she gets to the bottom of something..makes me admit things I had tried to keep a secret."

"Can't wait to meet her," Charlie laughed and she suddenly realised how much she'd missed in the last five years. "I'm glad to be back, you know."

"I'm glad you're back too," Kate smiled, "and you're welcome here for as long as you want. I'm just glad to have my sister home."

"Hmm, well looks like I came just in time. Now I get to see the Kate Beckett who swore she would never fall in love make that writer the happiest man in the world. But you better get a move on, Becks," she said, reverting to the nickname Maddie had used. "He might have waited for four years but he won't wait forever."

"I know," Kate said. She just had to find a way to tear down what was left of the walls she had put up so long ago. And figure out a way to bring up the developments in their moms case with Charlie.

A/N: I quite like this chapter :) there will be lots more Charlie and Kate talks - they do have five years to catch up on - but I'm gonna do the Castle and Kate chat next, seems they've got a lot to talk about too! Reviews more than welcome :)

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