[quick note if you are only familiar with the anime version of the canon story, a lot of stuff I am only hinting at or leaving unsaid (because if Nabiki didn't see it, it didn't happen in this story) will leave you confused as to what is going on. I suggest re-reading the manga version as well (volume 15 in English N.America market, vol. 17 in Japanese market) if you are confused.]

Nabiki was miserable. She had gone to sleep miserable, and she'd woken up miserable. She didn't want to get up. She didn't want to go to school, she didn't want to be alive. Okay, she didn't mean that last one, but still, she didn't feel like facing the world today. She huddled under her blankets in her warm bed and thought about last night.
She'd blown it, she knew she had. She had to let him go, it was the only thing left to do. Besides, if she had cried herself to sleep last night over him... poor Akane must be going out of her mind.
If he had any strong feelings for her, he would have said something last night. She hadn't told him everything last night, but she had told him enough. He hadn't rejected her, but he hadn't pursued her, either.
As she lay there feeling sorry for herself she heard a soft knock on her door. She thought of pretending to be asleep still, but knew that would just encourage more attention. So, with regret, she shattered the morning's silence and called out to whoever was knocking. "Yes?"
"Can I come in?" It was Kasumi's voice. She did a quick check and found that she was still wearing the clothes she had on last night. She rubbed under her eyes and found them to feel a bit tender and puffy. Lovely, there was no way Kasumi would miss all that. Well, there was nothing for it, if she told Kasumi to go away, that would just get her more of her attention. Better to just deal with her now. "Okay," was all she said in reply.
Kasumi opened the door to her room and stood in the door frame smiling her usual smile. She was convinced it was an act. Kasumi hadn't been like that before Mom died, and nobody was that happy and placid all the time. "I was just wondering how you were doing." Her older sister asked her softly.
She just looked at her older sister. Kasumi knew; she always knew. Kasumi closed the door and came and sat on the edge of her bed. She didn't move. She honestly didn't feel like dealing with anyone right now, even Kasumi. Her older sister reached down and started to stroke her hair, like Mom had done when they were little. The urge to cry rose up in her, but she summoned her iron will, and crushed that urge under it. She was done crying, at least for now.
After a while longer, Kasumi rose gently from her bed and walked softly to her door. As she left, she called out over her shoulder to her. "Whenever you're ready, if you want to talk, come find me."
She nodded at her older sister, and Kasumi smiled and then left with the soft close of her door. With that done, she rose from her bed and looked at the clock. She had overslept a bit. No wonder Kasumi had come looking for her. She honestly didn't care all that much. If she was late for school, then she was late for school. The way she was feeling today anyone crossing her path had better tread carefu—
A knock sounded at her door again. "What?!" She called out as she got more irritated.
The door opened, and there was Ranma. He smiled at her. "Hey, Nabiki, how are you doing?" He asked.
She quickly put on her professional mask and replied, "I'm fine, Ranma, how are you?" She noticed at that point that he was dripping wet. The fact that he was male at the moment must mean he had recently been splashed and then had hastily changed back with some hot water.
A dark cloud crossed his face and she could see raw anger in his eyes as he looked off at nothing. "I... I've been better." He said crossly before he evened out his tone and looked back at her. "So, see you at breakfast?" He asked her.
"S-sure." Was all she could think of to say.
"Great, see you down there." He said, again with a touch of anger in his voice. He closed the door again as he said it, and was gone.
I wonder what that was all about? She thought to herself as she stripped off her clothes from last night. She paused when she was done and considered trying to squeeze in a bath before breakfast, but another look at the clock convinced her that she didn't have the time. The gym showers at school would have to do later in the day.
She hurried and got dressed in her school uniform, and rushed downstairs. She didn't feel quite as bad anymore, but she refused to let her moods be controlled by anyone other than her. She was determined to keep control of herself on that front today, no matter what might happen.
As she entered the dining room, Ranma jumped up from the table and indicated the spot next to him as he smiled at her. "Good morning, Nabiki, here, have a seat."
Akane was in the room too. She watched Ranma with cool detachment on her face, but her trained eye was able to pick out the subtle winces and slight facial ticks that showed that Akane was both unhappy, and felt like she was guilty of something. Her little sister was hiding it well behind a mask of indifference, though. Akane ignored both of them and just continued to eat her breakfast at a steady pace, while looking straight ahead, just as she had done last night.
"You look beautiful today, Nabiki." Ranma said as he smiled widely at her. She looked sideways at him, and frowned.
It was extremely unlike him to say something like that to anyone, especially with witnesses around. Her eyes darted between Ranma and Akane, and she realized he was putting on a show to either taunt Akane, or punish her for something. Well, she supposed turnabout was fair play. She wouldn't join in, but she could accept the compliment gracefully. "Thank you, Ranma, you look nice too."
Akane twitched a bit when she complimented him in return, but she didn't say anything. She settled in and ate quickly. She knew she was running up against the clock. As soon as she set her bowl down, Ranma leaped up to his feet, grabbed his bag and her bag. "Get up, fiancée. Time to go to school."
"Huh?" Was all she could think of to say to that as she looked up at him in confusion. 
He smiled and reached down and offered her his hand. So, she shrugged and took it. He lifted her up like she weighed nothing and started to pull her towards the door. She looked back at Akane who was still eating. As he pulled her towards the door, she couldn't help but ask the obvious question. "But ... Akane?" 
Ranma continued to charge forward in his steady walk that was almost a jog and called back over his shoulder, "Leave her."
She frowned as she let him pull her along. It was clear that he was using her to get back at Akane, for ... something. Who knew what it was, it could be anything from a careless word that she had dunked him for, to her rejecting him as a fiancé again. Whatever it was, she didn't like being in the middle of it.
She looked at him sadly as he pulled her along, and decided what she would say when they spoke today during lunch.
They reached the school grounds without incident. Interestingly, Ranma continued to hold her hand as they walked onto the grounds. She didn't really want to be seen being led around by him, though, so she pulled on his arm and forced him to walk side by side with her evenly.
She slowed her pace to a leisurely walk and decided to just enjoy walking in the courtyard with him while holding hands. He wasn't in a chatty mood, but that was fine because neither was she. They were a dozen or so steps away from the front door when Akane ran past them at full speed. She didn't say anything to them as she ran by. Ranma didn't do anything to note her passing, except to hold her hand tighter. She sighed and walked with him to his classroom.
When they arrived, Akane was already seated at her desk, looking anywhere but the door.
Ranma's classmates noticed him approaching and called out to him as they saw him with her.
"Good morning, Ranma."
"So what happened?
"How come you're with Nabiki?"
She watched Akane as they greeted Ranma, and saw her twitching angrily as her classmates piled on with questions about her.
Ranma put his hand behind his head in embarrassment and stammered lamely with no explanation ready.
One of Akane's friends called out in disbelief, and interrupted him. "Does Akane know?"
Ranma looked down, still embarrassed, and muttered something unintelligible.
She smiled at that, despite herself. She knew he was using her to get back at Akane, but, that was exactly what she had done, at least a little bit, with him as well, so she didn't see the harm in it. However, she did have to get to her own classroom. So, she smiled up at him as she squeezed his arm and said, "Oh, you're so cute!" As she let go of his arm and dashed off she called back over her shoulder, "I'll see you after class!"
She made her way to her own classroom a short way away, took her seat, and tried to pay attention to her lessons. For the most part though, she sat through her morning classes in a daze. Her mind kept wandering back to what she was going to say when she saw him at lunch. And, if she was being honest with herself, she was also worried about what he was going to say to her.
When the bell chimed for lunch, she hopped out of her desk and hurried out of her classroom, and ignored her friends calling out for her and asking about her plans. She hurried back to Ranma's classroom and found him sitting in his desk, looking back sullenly at Akane as she studiously ignored him. Fine then, if he was still fighting with Akane, she could give him some ammunition. She called out to him in a cheerful voice. "Oh, fiancé Ranma!"
He looked over at her and perked up at the sight of her as he leaped over to the door to stand in front of her. When he did things like that, she was reminded of just how unusual he was. It was kind of funny that he could perform super-human feats like leaping across rooms like it was no big deal, but he couldn't handle simple things like relationships and dating.
Feh, who was she to talk, she obviously couldn't handle them either. Maybe those things were harder than training in the Art to leap across rooms.
Ranma looked at her expectantly and asked loudly, "What is it, fiancée Nabiki?" He looked back sullenly at Akane to drive home whatever point he was trying to make with her. With that out of the way, she walked backwards out of the room and waved for him to follow her.
She walked up the stairs with him to the roof in silence. Now that they were away from Akane, both of them had dropped their acts. At least they were now back in an arena that she was familiar with. She could dance the dance he was trying to lead them through.
Once they reached the roof and the door had shut behind them, she walked quietly over to the railing and leaned over it. He followed her a few steps but stopped short of joining her at the railing. She let the silence between them build for a while. When she finally heard what she had been waiting for, the sound of the door opening and closing again quietly, she put her plan into motion. "Ranma, what do you think about us breaking up?" She asked calmly.
The wind blew across the roof as she waited for him to answer. She knew Akane was on the roof somewhere, spying on them. When she hadn't been able to stay away the day Ukyo called her out, she knew Akane wouldn't be able to stay away if she called him out to meet with her right in front of her. She wanted to see how Akane would react to what she was about to do. And, she felt that it was the right thing to do for Ranma as well. It didn't matter how she felt.
"B-breakup? B-but why, Nabiki?" Ranma finally answered her.
"Ranma ... please..." she answered him in her theater-voice. "I know you really want to make up with Akane... and that I'm not the one you want." She spun around to look at him, and found him staring at her with disbelief.
Bringing Akane into it had the desired affect, though. He protested strongly. "Feh, please!"
She looked sadly at him. "You don't have to pretend with me. I'm setting you free, Ranma..." She smiled sadly to herself at what she was about to say. It should give him the push he needed to go back to Akane, and it should give Akane what she wanted. It was easier for everyone to just believe that she was an ice queen. She held out her hand to him and said, still in her theater voice, "... And for only ... five thousand Yen!"
He looked at her with wide eyes in utter surprise, then his eyes narrowed and he turned his head a fraction of a degree to either side, like he was searching for someone.
Ut-uh, I think he may be on to me. She thought. She wasn't really worried. If this didn't prompt him to go back to Akane, she could give him a harder push later. It was the best thing for ... almost everyone.
Ranma looked at her and said, "Five..." then stopped and walked to the railing himself. He leaned over it and was silent for a moment before he replied. "Forget it, Nabiki. I'm not breaking up with you."
She stared at his back for a moment, stunned by that. She certainly hadn't expect him to say that. She wondered how far he was willing to go to prove his point. She started walking towards him with a sly smile on her face. "If ... if you really mean that ... kiss me." She said sweetly as she arrived next to him at the railing and leaned in towards him seductively.
He started twitching almost immediately. "Huh ... but...why?" He stuttered in a panic as he leaned away from her. So much for his game face. 
She looked down and blushed a bit herself. She wasn't sure how far she wanted to push this, especially with Akane back there spying on them. She was trying to push him back to Akane after all, not drive a wedge between them. But, maybe Akane seeing him refuse her advances would be a good thing as well. So she pushed a little bit harder. "You say you want to be engaged to me... so prove that you mean it."
That was it, Ranma had reached his limit. He was twitching so bad she was worried he really would stroke out if she pushed him any further, so she retreated a bit and turned away from him and said sadly in her theater voice, "You can't, can you?!"
She turned back to face him and found him standing very still, and very pale. Well, that was it for him then. She'd have to sell it to Akane now, and hope for the best. She collapsed to the ground and buried her face in her hands and did her fake sob that usually worked so well when she was trying to control people. "So then you don't love me after all! You've been toying with my feelings all along!" She wailed.
Ranma tried to stutter something, but she was on a roll and cut across him. "You're only using me... to get back at Akane!" She then let loose with the most powerful set of fake sobs she could summon. After she let it sink in for a few moments, she stood up dramatically and put some heat into her voice. "If that's how it is... then forget what I offered you earlier! I'm not going to play your little game any more" She trailed off as she turned to where he had been standing, only to find it empty. He'd run for it. She couldn't say she blamed him.
For Akane's benefit she snapped her fingers and said something about a missed opportunity. She honestly had no idea what she actually said. She walked back to the railing and looked out over the city and then up at the sky. It was a beautiful day. The sun was shining, and the birds were singing. And she was miserable. She heard the door to the roof quietly open and close behind her, and she smiled sadly to herself.
Don't worry, Akane, you'll have him back soon enough. She thought dejectedly to herself. As she continued to look out at the sky, the bell chimed ending lunch, but she didn't move. She just didn't care at the moment. As the clouds moved over the sky and started to cover the sun a bit, she let a tear escape and roll down her cheek. So much for controlling her moods today as she had set out to do. She had never been more miserable.