You can try holding the mother down and seeing if you can get the baby to nurse off of her. She really needs something with a lot of energy and fluids right now, like milk. Is she still drinking? Usually if they are quite skinny and not moving much to eat or have diarrhea, dehydration is a major factor. You could put out some unflavored Pedialyte in a dish and see if you can get her to drink that. You might want to talk to a vet about doing some subcutaneous fluids. Unfortunately I wouldn't hold out for a good prognosis. At this age you see some of them start to fade if they have some underlying issues that weren't evident when they were younger, especially if she is already skinnier than the littermate.