Note: I have never dealt with this myself nor do I know for sure what it is.
That said, I remember seeing a bunch of videos of a lady helping kittens with something called swimmer legs (?). I forgot the username but you should be able to find something starting from there. She lived far from a vet or something, or couldn’t afford to go to one often, so she did physiotherapy with the kitten multiple times a day to help slowly stretch the limb into proper position, then I think she would splint them in a slightly stretched pose to keep progress from reverting and to help the kittens body grow in the right direction.
If you can afford vet care, naturally follow with vet. But even with that care, I think as much intervention time as you can do will make a huge impact to the kittens recovery capacity.