"The problems I am having are: she puts food in her mouth and won't swallow it, she will not use a sippy cup, she does not listen to directions at all, she will not try to walk, she is very jealous of our other foster baby, she does not like to be held."
The sippy cup usage and the not swallowing are issues you could have addressed by an occupational therapist or a speech pathologist. You might have to ask for a feeding specialist.
As for listening to instructions or trying to walk, you said she was mentally retarded. Retarded means slow, so all of her milestones and her level of understanding will come much later than her same-aged peers. She likely doesn't understand how to go about following instructions. Nor is she developmentally ready for walking. There are some perfectly healthy children who don't walk until 14 months of age. 14 months is not unusual to still be crawling or scooting when you have a mental disability involved. My two 3-year-olds are both still crawling, and neither of them used a sippy cup until nearly 2 years of age.
Have you contacted your early intervention agency yet? We have First Steps here in Kentucky, I'm not sure what your agency is in TN. Your social worker should know, or you can contact the health department. They can come to your home and do an evaluation to see if she qualifies for Occupational, Speech, or Physical Therapy, as well as sometimes Developmental Intervention. It sounds from your post like she would be a good candidate for all 4 of those services.
The bottom line is that her bahaviors are going to be more like that of a child 1/2 her age or less, even. Try looking at her that way and see if it doesn't lessen some of your frustrations.
It IS hard to live with children who are so involved developmentally, especially if you haven't been down that road before. Get educated about what to expect, and most of all get her the help she so desperately needs. I'm surprised if her worker knew she had these issues (PKU, etc) that she is not already enrolled in early intervention services.
If you need help getting her services going, PM me. I'd be happy to do a little digging and see if I can come up with contacts for you. 