She walks slower than her sisters, she rest's more.
She pushes the left leg out to get moving, once she's moving she is slow but she can get moving if need be.
it kinda reminds me of the cornish crosses legs whever they got too big too fast their legs were big like that and then they gave out from under them.
i would think about switching her food and maybe like you are doing seperate her and keep and eye on her .
is she real fat ? maybe she is too heavy ?