I'm just brainstorming here. When you had her in the sling, at any time were her feet touching the floor/ground? I wonder if she can be in it and you put your hands under her feet, if she will raise her body, pushing with her feet/legs? Maybe you could do this several times a day. Like therapy, with her only needed to support her body weight for a couple/few seconds at a time. If/When you notice any progress, increase strength, you could allow her to stand longer, say 5-6 seconds.
I'm no expert by any stretch of the imagination. I know she about 23-24 weeks now but she probably lost strength due to not using all of her legs to move. I did this with a chick with splay legs, both and totally out to the sides. I could not get the bandaid/tape on her, it kept coming off, so I tried my own therapy.
Please forgive me if this sounds stupid.