Does a mother have a right to keep a child from her father if there has been no custody decided by the courts yet?

Yes, but only if there is no order, temporary or otherwise, that states that the father should have reasonable access to the child. While custody proceedings are pending, there is typically a document issued that requires some sort of visitation with the parent that the child does not live with. If there is such a document, you don't want to violate it because it will hurt your chances for custody. However, if there is no document that gives the father either visitation rights or states that you must give the father reasonable access to the child, then you can do whatever you want until a court order is issued.