How can i help a child who had been abused?

my cousin, becky, is developmentally delayed and mildly retarded. she had lived in many different foster homes. becky had been abused and harred in some of them. she is now living with my boyfriend and i. we try to look after her as best we can. in the last foster home becky lived in, she was severely punished for vomiting. it made her develop a fear of opening her mouth. becky is afraid that if she opened her mouth, she might vomit and get punished again. becky had shut down and that is really not helping her to improve. i really want her to open up to us and learn as much as she can. what can we do to help becky?