Failure to launch: How can I motivate her to do the work to walk again?

The longer that your wife chooses not to walk, the more atrophied her leg muscles become and the longer it will take to get her back to walking. I'm sure they're already very atrophied since it's been a year already that she's walked. It will take a lot of hard work on her part to get those muscles back to working like they should and by the sounds of it, she is content to not have to walk.
Since your profile says that you and your wife live in an assisted living facility, the only suggestion I have to "motivate" her would be to threaten that if she doesn't start some PT, that you will have to place her in a nursing facility as you just can't care for her any more like she is, that it's just too much for you.
Perhaps that will start her rethinking things, and give her the motivation she needs. I wish you the very best.

I don't know that you can do anything to 'motivate' an unmotivated wife to walk again. When I had a full hip replacement about 4 years ago, all I had to do to recover was walk; first with a walker, then a cane, then without a device at all. That was it. No rehab, nothing. I recovered in short order and went back to work and driving my car in under 4 weeks.

If your wife was in rehab for 4 weeks, what did they do with her to get her back on her feet? I'm confused; you say she broke her arm, but now is not walking. If she's not careful, she's going to be fully wheelchair bound in short order and then what? I'd tell her that if she's choosing that alternative, then she'll have to be placed in Skilled Nursing b/c you won't be put into a position of having to lift her in and out of a wheelchair 24/7.

If she's faced with life in a nursing home OR learning to walk again, that may be the 'motivation' she needs to get up off of her arse and do it!

Good luck!

Having been in rehab I would imagine that you have all done all you can to motivate Mom. The MD has gone the route of steroids, but does sound now as though the pain is severe.
I would discuss with Mom and MD the possibility of trying some low dose anti-depressants.
I know you have been trying. The MD is trying. The rehabs try very hard usually.
The sad truth is that there may be little you can do to motivate her. Many end their days wheelchair bound.
I am sorry and hope others have some possible answers that bring more hope.