foreach ($xls2cli->Region as $Region){
    if( $Region['id'] == 'Region1' ) {
        echo $Region['id']."<br>";
        foreach ($Region->site as $site){
           echo $site."<br>";
        }
    }
}

foreach ( $xls2cli->xpath('/Region[id=Region1]') as $Region ) {
        echo $Region['id']."<br>";
        foreach ($Region->site as $site){
           echo $site."<br>";
        }
    }
}