AN: This is quite addictive, I like writing a new AU each day and it helps me relax and unwind after a difficult day at work. I hope you enjoy this one.

Best Friend's Sibling AU

One thing that Brienne and Tyrion had agreed on from the moment they met was that their lives had effectively become a sitcom. They had shared a flat in their university halls of residence and apart from Tyrion's dwarfism and Brienne's height; they also had Renly and Loras, the lovable gay couple who got together that first night of Fresher's Week and Daenerys; the silver-blonde bombshell who had half of the campus falling at her feet. Thankfully their flatmates agreed and it was something they all played up for fun; assigning each other certain roles from famous sitcoms and shows. Whether it was Daenerys' terrible love life or numerous drunken misadventures or the many times they sat up laughing until they cried; their lives always seemed to have some sort of sitcom trope going on.

After their first year; all of them had moved out of halls. Renly and Loras had moved in together, Daenerys moved in with a few of her friends, Yara, Missandei and Grayson (who preferred to be called Grey). Although they all remained good friends and hung out a lot; Brienne and Tyrion grew closer to each other than they had to the others. Tyrion and Brienne had found their own place and their friendship moved from strength to strength. From lazy days watching trashy TV to panicked study sessions, attempts to make cool meals that looked amazing on SpiderWeb but almost burned the kitchen down, nights when they sat up having deep, personal conversations that they had never had with anyone else and fun nights out with the rest of their group of friends which usually ended with some sort of trouble the next morning; they managed to build a strong friendship. Both of them were honest, clever, an obscure sense of humour and had had a lifetime's worth of people treating them like crap. Tyrion was definitely Brienne's closest friend; which is why this current situation was not ideal.

Brienne was dating someone.

Except it wasn't the obvious problematic scenario of two best friends with such a close bond; Tyrion wasn't in love with her. Instead Brienne had done something worse.

She had broken the unspoken rule of all best friends: A best friend's sibling was off limits.

She was dating Tyrion's older brother Jaime.

It began the day Tyrion had discovered Shae cheating on him, as in actually catching her in the act of blowing some random guy, and Tyrion ending the relationship on the spot. Brienne was meant to comfort him, which for them meant getting drunk watching stupid horror movies; the stupider and gorier the better. On this occasion however, Tyrion's older brother Jaime had rushed down to their college town to offer his brother some moral support the moment he had heard the news. Jaime also provided some more expensive alcohol than what two students could afford. The night had started out OK; they watched some stupid horror movies filled with fake blood and bad acting. They let Tyrion vent his frustrations about how his relationship ended complete with Jaime and Brienne's foolish and immature plans of revenge and Jaime even ordered them a takeaway from Hot Pie's. His treat.

Around midnight, Tyrion had decided that he needed to get some sleep and after he had stumbled off to bed, Brienne and Jaime had been left alone together. It wasn't the first time they had met but it was only a handful of times that they had been able to have a conversation that didn't include other people or have been yelled across a nightclub dancefloor. Their conversations normally involved some sort of banter or Tyrion or arguments over their shared favourite TV show; Clash of Crowns. Finding each other slightly tipsy but in a serious situation normally would have been a source of awkwardness for them but tonight had been a very different night.

"I still can't believe she did that." Brienne said as she filled her drink. "Tyrion did so much for her. He helped her when her student finance messed up, helped her with her exam revision and essays, took her to Dorne for Valentine's Day and for what? Her to be messing around with some other guy?"

"He deserves better, he's had to deal with so much of this. People think because of his disability; they have the right to treat him like shit." Jaime said darkly; he had always looked out for his brother. Tyrion was a flirt but he knew that most of the girls he chased were either interested in his money or the ability to claim they had slept with him. Shae was supposedly different but really, she was no better than any of the others. "She has no idea what she lost."

"Well Daenerys, Yara and Margery all offered to help me track her down and beat her up." Brienne said, scrolling through the girl's group chat to show Jaime the messages of support and plans to drag Shae across campus by the hair and introduce her to Yara's archery kit. Or take Tyrion out clubbing with them on Friday night to find him someone better.

"You're good to him," Jaime said sincerely. He had been aware of the two's friendship and he had liked Brienne from the start. He could see that she was an earnest and true friend to his little brother, plus she could keep up with their banter, loved Clash of Crowns as much as he did, including all the fan theories and discussions, plus Brienne could drink like a fish. That was enough to make her perfect to the Lannister brothers.

"He's good to me. He's my best friend." Brienne said as Jaime moved closer. In the past few seconds, there had been a chance in the room, a notable shift in the tension. They both knew what was going to happen within the next few minutes, the only question was how was it going to happen.

"You've always looked out for him, he cares about you a lot." Jaime said quietly as he moved closer to Brienne, the alcohol making him brave. "And so do I,"

"What are we-" Brienne began before she was cut off by Jaime kissing her. She kissed him back, deepening the kiss and lying back on the couch. Jaime lay on her and as they kept kissing began to slide his hand under her shirt and up her back. Reality hit and Brienne broke the kiss. "Wait."

"What?" Jaime asked, barely moving his head away from her. They were still so close that he could kiss her again.

"We can't do this, we're drunk and come on; we can't do this to Tyrion. You're his brother, I'm his best friend." Brienne explained as she helped remove Jaime's hand from her back, ignoring the loss of warmth it caused her.

"I know but I mean it Brienne. I care about you. A lot. I-I never just saw you as my little brother's best friend." Jaime said sincerely as Brienne tried to think. On the one hand they were drunk and they would probably regret it in the morning. On the other hand, she had never seen or heard Jaime be this honest before…

"Just tonight." Brienne whispered as Jaime grinned cockily at her.

"For longer." Jaime replied as he stood and Brienne led him into her room.

By the time Tyrion awoke the next morning, hungover but with a phone bombarded with messages of support, Jaime was still asleep on the couch and Brienne was making breakfast for them in the kitchen. Tyrion had smiled wryly to himself as Brienne handed him a mug of coffee. He was lucky to have some people who cared this much for him.

In the month since that night, Brienne and Jaime had been secretly dating. They messaged and texted each other every day and had managed to see one another on days Brienne had free or at weekends. They had gone to the cinema, gone to dinner or just spent hours talking about everything and anything. They were certain about how they felt for each other; this wasn't just a crush or a fling, this is was real. There was only one problem.

"We need to tell Tyrion." Jaime said as they walked back to Brienne and Tyrion's flat, hand in hand. He had come down to see Brienne but had not told Tyrion he was there. He did feel bad about keeping the secret from his brother. They had nothing to hide or to be ashamed of. Tyrion would understand. Eventually.

"You're right, we have to tell him." Brienne said; if her life was a sitcom then it had resorted to the oldest trick in the book. Falling for her best friend's brother. How clichéd was that?

"Just so you know, if he kills me, you can have my DVD collection." Jaime said with faux-seriousness. Brienne laughed at him and as they walked up the stairs to the apartment, she gave a soft sigh. She was not looking forward to this. Would Tyrion be hurt by them? Would he feel betrayed? Or pushed out or overlooked? Would he go absolutely insane?

"Better get it over with," Brienne said as Jaime stroked her cheek.

"Whatever happens, I will not and will never regret the past month with you. You're everything I've ever wanted Brienne and it actually scares me how much I care about you." Jaime said truthfully. So much had changed in this last month, he was feeling things that he had never felt before, never felt this intensely before. He was thinking of their future, maybe even their forever. Yet a month ago, Brienne was just a friend he saw on nights out with his brother or who he shared memes with after the most recent Clash of Crowns episode. It should have scared him, instead he had never been this happy.

"I feel the same," Brienne replied. It was true. She had never felt this way about anyone before. It just felt right, like they were meant to be. It wasn't just the honeymoon stage of a relationship; this was deeper than that. More than just spending time together and flirting and sex. Whatever this was between them, this was something more intense than any other relationship they had had. Maybe even something that could last a lifetime…

As they reached the door, Jaime turned, put his arms around Brienne and kissed her. The rest of the world faded away, the way it always did when they kissed each other. They weren't sure for how long that they stood there entwined. They had been so caught up in each other that they hadn't noticed the door opening until they heard a pointed cough. Brienne and Jaime broke apart to see Tyrion standing in the doorway, an eyebrow raised as he looked at the two of them intently. For a moment, none of them moved or spoke until Tyrion raised the mug in his hand as if making a toast before walking into the flat again.

Busted.