use get_page twice like this handy little function
function get_grandpapa($page_id){
$current_page = get_page( $page_id );
if ($current_page->post_parent > 0){
//has at least a parent
$parent_page = get_page($current_page->post_parent);
if ($parent_page->post_parent > 0){
return $parent_page->post_parent;
}else{
return false;
}
}
return false;
}
This function returns the grandparent page ID or false if there is no grandparent.