ya lo he arreglado
Código PHP:
$select_ofertas_3=mssql_query("select * from familias where fam_codi in (select fam_codi from articulo where art_codi in (select art_codi from ofertas where cli_codi = '0001' and (ofe_inicio <= '$tiempo_hoy1') and (ofe_fin >= '$tiempo_hoy1')))");