[The first 10 chapters of this story take place entirely in the canon for the "Hand-Me-Down Ranma?" Episode/Manga arc. I supply an internal monologue for Nabiki, and additional scenes in between, or extending, the canon scenes. All of that is done to flesh out where I want the story to go, and Nabiki's character and motivation, which I think are IMPORTANT. However, if you want to skip straight to the point where I leave the canon behind, chapter 11, at the end, is where that happens. If you do that, though, you will probably be a little bit confused about how Nabiki is acting. But you can always circle back to the first 10 chapters if you want to fill in that information ^_^]

Nabiki barely had time to realize that Akane had just destroyed the balcony they were all standing on, before she felt herself falling. Before she had time to panic, though, she felt someone strong scoop her up and shield her with their body from the falling debris. As they softly landed, she looked up to see Ranma holding her.
Of course it was Ranma, who else would it have been, Akane? While she knew Akane was actually a very kind girl, when Akane was angry, she didn't act like her usual self. Ranma, on the other hand, seemed to always have an eye out for protecting those around him.
She looked up at Ranma as he held her. He was very strong, she had to admit. While she didn't really mind him holding her, she knew that Akane seeing him holding her would cause ... problems. She felt that she at least owed him an acknowledgment of his actions though. So she thanked him. "Well, it looks like you saved my life, Ranma. Thanks."
Ranma looked down at her and smiled nervously. "Yeah, that was a close one..."
She knew Akane was standing there watching them. So after a moment had passed without him putting her down she tried to prod him again. "Maybe you shouldn't have, Ranma." She stared pointedly at Akane to try to drive her point home. Honestly, the boy could be so dense. When he still didn't put her down, she tried again, this time a little more bluntly. "I mean, saving me and letting your fiancée fall..."
Ranma clued in at that, and hastily set her down. "What was I supposed to do? You're a normal girl ... unlike Akane." He said as he glanced nervously over at her sister, who was staring at them with an outraged look on her face. He walked over to Akane as she smiled sardonically to herself. This wasn't going to end well for him.
As he talked to her sister, she tuned them out as she usually did, and was getting ready to head back inside and tell Daddy that the balcony was broken. She was also trying to decide how she felt about Ranma holding her and how it had made her feel. She decided that she had liked it. Ranma was only a year younger than her, and he was gorgeous. She really didn't mind him holding her, at all. 
Akane raised her voice and yelled something then that snapped her out of her revere. "That does it, we're finished! Why don't you just go get engaged to Nabiki?!" Akane's yell was accompanied by the sound of a slap. What her little sister had said had grabbed her attention. She had to admit, at least privately to herself, that she sometimes regretted hastily foisting Ranma on Akane.
When he had showed up after Daddy had announced out of nowhere that one of them had to marry him, her self preservation instincts had kicked in. At the time, she had done what she had felt she had to do to duck a possible major liability. Now, after she had been observing Ranma for months as he took care of Akane, she had to admit that having him devoting his attention to her wouldn't be ... all bad. He was kind to all the girls after him, actually. She had watched him do what he could to spare the feelings of the rest of the girls chasing him, while still trying to live his own life and become a world-class fighter.
Of course, she would have to make him work for it, she wasn't interested in being someone's toy girlfriend or trophy wife. Of course ... there was still the question of Akane.
She looked over at the two of them, and sure enough, Ranma was now sporting a red hand print on his face as he twitched and stood in front of her sister. Oh Akane ... she really was a sweet girl, but Ranma really seemed to be able to push all of her buttons. They just didn't really seem to be a good match. Neither one of them seemed to be able to give the other what they needed.
Poor Akane. Mother's passing had really messed her up. In truth, it had messed them all up. Kasumi had done her best to step into the mother role, which was extremely awkward to watch as Daddy more or less treated her like the woman in charge of the house. But Akane ... Akane had known Mom the least amount of time, and she had also clearly needed her the most.
As for herself, even when she had been a little girl, she had prided herself on being self sufficient. Akane tried to pretend she was as well, but it was clear to anyone who watched Akane, that she not only thrived on outside validation, but that she needed it. It wasn't enough for Akane to do something and take pride in her own accomplishment. Like a child, Akane needed someone to pat her on the head and tell her she was a good girl and had done a good job.
Akane's emotional problems also could be traced back to the passing of their mother. Daddy's slow meltdown into how he acted now, hadn't helped either. Yes, Akane had been a tomboy even when Mom was alive, but not like she was now, and Akane hadn't had the out of control temper then, either. 
She knew that Akane thought she was in love with Ranma. But she also knew it was the fantasy love of a child. Akane was more in love with the idea of being in love than anything else. Akane was not ready to be in a real relationship with anyone. Her little sister's selfishness when dealing with Ranma was readily apparent to anyone observing them together.
Akane clearly thought that the role of the girl in a relationship with a boy was to allow the boy to shower them with praise and adoration, and that the boy should never admit that the girl had ever done anything he didn't agree with. Akane had no concept of the give and take of a healthy adult relationship between equals. Watching her with Ranma was like watching a little girl trying to play house with some bewildered boy she had captured off the playground, and getting upset with him when he didn't follow the rules of her made-up game.
Ranma did his best to keep Akane happy when he wasn't deliberately doing things to spite her because she had pushed his buttons. But, it was clear to everyone who knew them, that Ranma wasn't all that much more mature than Akane was. Anyone dating Ranma would have to help mature him a bit if they wanted the relationship to succeed.
She thought about it, and decided that she would be willing to do that, if Ranma reciprocated. She looked over at Ranma and decided that he could be a worthy project. Still ... she supposed she had better let Akane have a chance to retain her claim on him. It was only fair. If Akane decided to be childish about it, and she would not bet against that, then so be it; she would accept the change in engagement and see if she and Ranma might be a better match. Even if they weren't, they might at least have some laughs for a while.
Her decision made, she put her hand on Ranma's shoulder and replied to her sister. "Akane? Are you sure?" She said as she stepped closer to Ranma. Ranma wasn't paying any attention to her, he was babbling something to Akane about her not meaning what she said.
She cut across him. "If you say take him, that's it, he's mine." She grabbed his arm possessively as she said this and leaned into him. She could feel him tense up as her words registered, but she didn't take her eyes off Akane. Akane had turned back from the dramatic exit she had clearly been planning, and looked back stricken with shock at her.
Akane had the greater claim on him. So she decided she would back off and let Akane take him back, if Akane took back what she had said, now. Akane had put in the most time with him after all, it was only fair. But she wanted Akane to know she intended to take her up on this if her younger sister didn't take him back.
"I'm not borrowing him. I'm keeping him. Understand?" She smiled as she grasped Ranma's arm tighter to drive home her point. "And you're not getting him back!" Ranma looked down at her with naked panic in his wide eyes. She knew he must be thinking she had either lost it, or was teasing. She smiled reassuringly at him before turning her attention back to Akane. She knew exactly what she was doing. She hoped Akane did as well.
Akane narrowed her eyes at her and stomped over towards her. She briefly wondered if Akane intended to hit her. Instead, Akane threw a ribbon on Ranma. "Take him, the creep's all yours!" Akane said angrily as she spun on her heel and stomped off to continue her dramatic exit.
"Thanks!" Ranma called out angrily to Akane's back.
She had not let go of Ranma's arm yet. She was pressing herself up against him still. She realized that it was very forward for her to be doing what she was doing. Granted, she had done it mostly to pressure Akane to take back what she had said. But, now she was curious what Ranma would do about it. She looked sweetly up at him, and waited. Ranma looked between her and Akane's retreating back with rapid turns of his head and growing panic on his face.
"Relax, Ranma. It's over. She'll calm down eventually, but even if she doesn't, it doesn't matter. You're mine now." She said with a sweet smile and her best seductive voice as she continued to press her body into him. He smelled like cedar wood and some sort of a flower. She'd never noticed that before. She could tell he was about to stroke out so she stopped torturing him and pushed off him lightly as she smiled up at him. "I'm just teasing you, Ranma."
"S-so, you didn't mean it about ... about the engagement stuff?" He asked.
"Oh, no, I meant that." She replied, still in her sweet voice.
"Nabiki—" Ranma started what was sure to be a long conversation, so she cut him off, she didn't really want to get into it right now, especially not outside in the cold, especially since she herself wasn't sure how serious she was about him. She could feel a growing pit of warmth inside her, though. She had enjoyed being in his arms, and holding on to him just now.
"Ssh, don't worry about it, Ranma. Our fathers just want you to marry one of us. Akane gave you up, fair and square, and I have accepted the engagement. They'll be fine. Akane will sulk about it at first, but I think she'll get over it pretty fast, and she will eventually realize that she was never ready to be engaged to anyone. Eventually she will probably thank me for agreeing to take you." She said as she pressed a finger to his lips. Then, with a grin, she reached up on her tip-toes and kissed him on the cheek. She ran into the house then, and left him standing out in the garden, absolutely speechless.
She walked calmly into the entryway. She fully expected Akane to be waiting for her. She decided that if Akane asked her nicely, she would give the engagement back to her. If not, she would make her pay to get him back. If Akane didn't make a serious attempt to get him back, though, she really would keep him.
To her surprise, she did not find Akane waiting for her when she entered the house. Akane wasn't waiting for her on the stairs or in the hall outside their rooms, either. She opened the door to her room cautiously, but Akane wasn't in there waiting for her either. So, she shrugged and closed the door behind her. On a whim, she pulled her curtains back and looked out in the courtyard where she had left Ranma. He was still standing where she'd left him, in the exact same pose, like he was frozen.
Oh well, he'll snap out of it eventually. She thought happily to herself. She let the curtains fall closed again with a soft smile to herself. After she got undressed and changed into her pajamas, she sat at her desk and began writing out some plans.
If she was going to be engaged to Ranma anyway, she might as well have some fun with it. Akane had been having all of the fun with him so far. Even if Ranma didn't honor the engagement, tomorrow would be ... interesting. She hummed to herself cheerfully as she worked. She was looking forward to tomorrow.