a child who refuses to bathe is often a formerly sexually abused child.
Nevertheless, she needs to learn that you will keep her safe.
You will probably need to be in the bathroom with her to get her to shower. I would offer to "help" if she needs it (that is probably the last thing she wants!)
I have raised three attachment issue children, and although I have not been a roaring success at it, anything I have learned is available to share.
Treat her like a very young child and don't allow her to have much control , if any. Attachment disordered children want to be in control because they are unable to trust, but any control they get only makes them sicker. She needs very strict parenting and needs to learn that you are strong enough to keep her safe and will not abandon her.
Get her into therapy with an attachment therapist while she is still a foster child because they will pay for it.
If you have this child because you are wanting to adopt her, I would be very careful; I thought the adoption finalization at age 9 would help my daughter attach better,l but it did not and she very nearly bankrupted our family with trying to get her the help she needed. She never did heal, nor take any responsibility for her own life, and as an adult she cannot keep a job for more than a few weeks before finding a reason to quit.
If I can be any more help, e-mail me.
L.