I live in Oregon and while we have some pretty stringent standards for adoption as a whole, they do NOT include being required to be a "foster parent".
Two things I would recommend...1) retain an attorney (one who specializes in Juvenile Dependancy) and petition the courts for temporary placement and 2) contact your nephew's attorney directly.
Also...have the child's parents' "parental rights" been terminated by the courts or are they surrendaring them volutairily?