I get it, Star. You described it very well. If you ever get the chance to look at a model of the neck (bones, nerves and blood vessels) you will see how it looks like an intricate puzzle with little tube spaces here and there for nerves and blood vessels to fit through, like multi-coloured spaghetti. The problems come when you get those tubes in the bone collapsing for whatever reason, and pressing on whatever is trying to fit through the tube. If yo over-compress a blood vessel, it doesn't do its job well, of carrying blood from this place to that. And blood vessels carry good blood (nutrients, oxygen) to various parts of the body, as well as waste products away from various parts of the body. If the blood flow is impaired, then the cells 'fed' by those blood vessels can become toxic, starved and die. Nerves also are important - if they're blocked or swollen (and right now, there will be nerves in her neck area that are swollen, but this will ease) then they don't feed electrical signals to the muscles so well, and this over time can lead to a reduction or loss in function. Nerves are more easily permanently damaged. There are a lot of things that can be done to compensate for reduction of nerve function, but hopefully your sister won't need any of that.
I predict that in two weeks she will be feeling a vast deal better. In two months, exponentially more so. It will take time and she will probably have some problems that will continue for longer, but all improvement will be good. From here, the only way to go is up.
My nephew broke his neck in a school football scrum. He was a soccer player who decided to play ONE game of rugby. It was also his last. WHen the scrum collapsed on him, he was lying there unable to move. They air-lifted him from Newcastle to one of Sydney's best spinal hospitals and operated on him to stabilise the break. They said it was his strong neck muscles form soccer-playing (heading the ball) that saved him. The muscles cradled the neck and stopped the spinal cord form being severed. A week later after surgery, I got to see him take his first steps. He had a halo on, had to wear it for a few months while bones knitted. Wires wrapping around him like a roll cage for his head and neck. That was many years ago - if you are able to see any of the wedding photos on my FB site, you might see this very handsome blonde man who jumped off the back of the boat to push us off a sandbar. He'd be about 37 now. Still a model of physical fitness. I know he took about a year to recover enough to go back to training.
Marg