There are numerous reasons why a foster parent can keep the truth from the biological mother. For instance, if the foster parent is abusing the kid (s), there is no way they will tell the real mother about it. Again, some foster parents have this notion that the biological parents are interfering with their lives. Talk to the dad or your caseworker.
If the biological mom has a caseworker, she should tell her of her concerns, as there could be many reasons why the foster mom isn't telling the bio mother certain things. The caseworker needs to find out whats going on. I recommend that the bio. mom retain a good relationship with her caseworker and the foster mother, so communication can remain open. It is all about the child whether being cared for by a man or woman.